2005, 161,000 miles, red triangle and problem notice

I'm an original owner of the car above and haven't had any issues (except the paint peeling).
After leaving the car sitting for 12 days while I was away, I returned and the car was fine the first night.
Second day the triangle came on, which I've never seen. After taking it my personal mechanic (who I trust), he recommended Toyota and then call him with the diagnosis. Our Toyota was recently taken over by Autonation and twice they've lied and tried to oversell to me. My mechanic has seen a lot of this lately so he and his wife look out for their customers.
So, Toyota says it is the main battery and will run me approximately 3500.00. I was not prepared to spend this, but wanted to keep the car for my now just turned 16 year old.
After learning about this forum, I've done some research and think I found a place that will replace the battery, test it, warranty it for 1-3 years and would be under 1500.00 (likely I need the drive belt and water pump changed, too).
Seems worth it to me. Any suggestions?

 

So now I'm at 207,000 miles, with the teen daughter driving it around town to work and college. I did end up replacing the HV battery a couple years ago for 1200.00 (refurbished w/ 3 year guarantee). Car has been great. For 6 months we've had the ABS and brake light on which multiple mechanics told me is fine to drive with while Toyota said it would be 3000.00 to fix 8 months ago.
Today we took it to the mechanic as the red triangle came on with the engine light. The fuel tank has gas but is not registering it and the last cube keeps blinking.
Mechanic advised leave it and monitor gas usage or replace for 500.00, but it's ok to drive.
Opinions? We are trying to hold off buying another car for a bit longer.

 

It's at the dealer today getting the actuator replaced (yes, warranty item!! and they said the main battery (3100.00)is missing some cells and my water pump needs to be replaced (530.00).
I found a business that will come to you and replace the battery for 1000-1700, depending on which warranty you go with and since I did that once already and got 4 years out of it, I'll try it again.
My regular mechanic will recheck the water pump (dealer has told me this needed replacing urgently every time it's had a check in the past 3 years, then my mechanic says no, it's fine).

 

I had the battery replaced on Saturday (unlimited mileage replacement) and the water pump repaired.
Now the car turns on but stays in neutral. The mechanic is trying to recharge the 12V and see if that is the issue.
Another Prius person recommended the power button could be worn or a sensor on the brake.
Anyone have something similar?

 

The mechanic disconnected and retried the 12V and charged it. Speedometer doesn't come on and car won't shift to anything except neutral. He's tried multiple times and can't get the car going.
A couple times in the past year the display went out but Toyota said it wouldn't effect operation of the car.

 

I think it was the middle panel I told them about but I'll circle back with Toyota. Our local dealer has lied so many times I'm not keen on going to them for anything.
I'd really like to get the car running again this week. I've been pretty happy with Pep Boys lately, but need the car to turn on and drive (it did once this morning for about 2 minutes before the red triangle appeared).

 

The main battery was replaced Saturday and car ran for about two minutes. Stayed at the mechanics so the water pump could be replaced Monday. Since then the car will turn on but won't drive. Speedometer doesn't come on and stays in neutral. Codes are C1259 and C1310.

 

Any other ideas for getting the car at least driving? Codes are C1259 and C1310.

 

Just saw my mechanic and we are wondering if the ABS/Actuator work done at Toyota missed a brake sensor as the lights are all on for ABS. The car was driven from Toyota to the mechanic (3 miles) where it had the main battery replaced, after a week (delay with original company I had planned to use) and then driven for 2 minutes to make sure the battery was good. The car stayed at the mechanics since the water pump also needed to be replaced, which he did on Monday. Car continues to turn on w/ radio etc. but same lights as when I dropped it at Toyota 2 weeks ago. I called Toyota and they said have it towed back. We'll see what they say.