Is there such a thing as a Prius being too old?

I have a series one 2002 Prius which I bought 2cnd hand 10 years ago. It was an ex Govt (Environment Protection Dept) car when I bought it with 56k on the clock. It has been a fantastically reliable car for 10 years. It had full log book service till 190K and was due for the 200K when I lost my job 4 years ago. I could not afford the full service and ended up with just a basic service. Unfortunately it has had Zero servicing since, apart from the occasional engine oil level check and top up. On Boxing day morning I was greeted by a Christmas tree of warning lights, including the "triangle of death".
It took me quite some time to discover what the 'Inverter coolant reservoir' was. It had a tiny bit of what looked like transmission fluid or power steering fluid in the bottom. About an inch below the "add" mark.
Yes.... I know, I know... and all very sad.
First point is that I have not driven it since. I have started it and turned it off pretty quick when I heard a suspect noise. Actually thought it might have been a timing belt, but no.
I have looked through a lot of posts on this forum and watched a lot of videos re Inverter coolant and coolant pump. I eventually topped the reservoir up with demineralised water from the local servo but the pump does not seem to be working. Well there was a little movement and I could feel the vibration. The other thing is the liquid looks very thick. Sort of like Auto trans fluid, which is what I first thought it was. When I squeeze the hose leading back into the reservoir I get very thick fluid coming back into the reservoir.
I have rung the local Toyota service dept, which only opened this morning after Christmas and need to ring them back when they quieten down. My Partner's Son in law's friend is a mechanic at another Toyota dealers and his advice was that it needs to go on their computer to check the error messages. Also that it is folly to try to bleed the inverter fluid yourself as you will never get rid of air bubbles, which is the main cause of inverter failure.
My 2 questions are:
1) How safe is it to drive the car to the dealers as is?... about 10km.
and
2)At what point would I be best advised not to throw good money after bad?
The car has an agreed value of about $5000 on insurance and I would guess it's going to cost about $1500 for Inverter pump replacement and full service. That in itself would be acceptable if it meant the car would continue to give more trouble free miles for a long time but I fear it may just be the tip of the iceberg.
I would greatly appreciate hearing from any one with an older Prius with high milage. Especially anyone doing basic maintenance themselves. I am unemployed and still 18 months off the age pension. I actually use the car as a work horse, delivering junk mail etc. It's far from the shiny new(ish) vehicle it once was.

 

Update........
Gave up waiting for the local dealer to get back to me so rang the other mentioned dealer across town. Ended up with a pretty good arrangement which is a bit "best of both worlds". I have the car booked in next Tues afternoon and the mentioned family friend is going to do an inspection and report with his recommendations and will be doing any work I choose to go ahead with. May also be able to work out some sort of maintenance plan if I am to keep the car on the road, whereby I do some basic stuff like oil changes etc and let him look after the more complex stuff. I have basically done all my own maintenance for years on past cars but always felt this car to be too complicated. I do minimal driving these days, less than the 45k (kilometres not miles) in 4 years would suggest. I have been thinking of living without a car for some time as I have a good bus service outside my door which is free to use while the car costs me $25/week in registration and insurance alone. One option would be to simply put it off the road for a while with the option of putting it back in the future.
A lot depends on the outcome next tues but anything I can learn about stretching the life of the car out at manageable costs in the mean time would help enormously with any resulting decision. My biggest concern is throwing good money after bad and any talk of thousands of dollars to replace batteries is just the type of thing I'm talking about. Do these cars still function with dead and/or dying batteries? or do they just spit the dummy and shut down?
I will have a good read in the series one forum and I am most interested in just how much I can get away with doing myself to just keep the car on the road in a safe and usable manner.
OH and Kenny, the reason I mention the "insured value" is just to give some idea of local value, which would of course be the estimated value of the car in good working condition as a write off value. Of course that would then be less excess. That value reduces each year as the policy renews.
Thanks for all the advice and wish me luck next Tues, not just with the outcome of the checkup but with actually driving the car there in the first place. The more I think about it the more I think the problem with the Inverter coolant pump may be more a result of failed coolant rather than a failed pump. It may make sense to change the pump anyway though. I will be happy to just go with what Jason recommends. Nice to take a car to a dealer and actually feel you can trust what they are going to say.

 

Update
Thank you all for your replys and there are a couple more I had missed. I am very lucky to have a family friend who is a mechanic at a Toyota dealership. I took the car in for a check and the main problem is the HV batteries. At least according to the scan. Looking like I am heading to life without a car as a replacement that is not just a whole heap of "someone else problems" is out of my price range. Situation is at present the car is 'drivable' with the warning lights on but dying. End result being batteries giving up completely and car refusing to start or function. At least that's my understanding atm. I have joined the local RAA for towing insurance on a monthly basis and I'll see what happens. There are a number of things I am not clear on but it seems to me the HV batteries from a Series One are not interchangeable with later models. I had half thought of buying the new one (as long as it comes with a decent warranty) thinking well if worst comes to worst I could sell it on. That got me to thinking maybe I could find one from a wreckers. Big question is what fits what though.
According to Jason ( the mentioned mechanic) it would cost me about $3200 for the battery including fitting ( which is about $400 labor and having watched some youtube videos nothing I would attempt myself) He also suggested getting a basic service in the first instance as that would give me a report on any potential problems. ( probably about $250)
A full service, including all fluids, spark plugs etc etc would probably be about $1000.
Now here's the thing. OK. I could probably offload the one I have to a wreaking yard for next to nothing and spent that money on something else but would I just end up with someone else problems?
Btw the coolant pump does appear to be working, though not that energetically once the car warms up. For an extra few hundred $ it would probably be wise to replace it though if I get all the fluids changed.
I guess the bottom line is that if the HV battery for a series one fits a series one only it would be absolute madness to buy a new one.