Questions on Installing 2nd battery for Ham Radio

I have read a lot of other information out there on installing a VHF/UHF Ham Radio in a Prius already. Some have said they ran it directly to the small starting battery in the Prius and had no problems, others have said they installed a 2nd battery. I want to be able to operate on high power which is 35-50W on UHF and 50W on VHF when I need to. While most of the time, I use just 5W there are times where I need to go a bit higher.

I have several questions here. I really think connecting a 2nd Sealed Lead Acid AGM type battery located in the plastic tray under the back hatch area is the best solution for me as this will ensure I am never dragging the car's battery down to a lower voltage state during a long rag chew etc.

I have looked at this guys install, see web link:
2005 Toyota Prius Mobile Installation

Looks like this guy has a really nice setup. However I was also looking at this device to use between batteries:
125 Amp Voltage Sensitive Relay (VSR) - Voltage Sensitive Relays - BEP Marine

I don't know a whole lot about Isolators other than the fact that I think I should have one for my install.

What I want to do, is have a "Radio Battery" for the Ham Radio only, and have this charge directly from the cars charging system when the car is on. When the car is off, my "Radio Battery" should supply power to my radio, while not taking any load off of the Prius starting battery.

I would like to hear some input from both people who have done this, and from people who are very technical about the best way to install this.

Some other questions:

Is the standard 2008 Prius starting battery a wet cell battery? I noticed it has a vent tube to vent the battery. If I use an AGM sealed type battery, what will the different chemistries do if hooked up together with a voltage sensing relay/isolator do? Do I need some kind of 3 or 4 stage charger for my "Radio Battery" and if so, who makes them, (post links to product).

Also, while doing a test of the Prius starting battery, I found that while the car is off, the battery is about 12.7 volts give or take. When the car is on, it is 13.89 volts (with engine off) and the same with the engine on. This tells me that it is charging from the hybrid battery. What is the charging rate that the Prius supply's to the Prius starting battery and how many amps can it supply from the Prius at maximum load. I'm trying to figure out if I had lets say, the Car Radio on, a few other 12V devices in the 12V outlets etc. at what point am I drawing more than the car can supply and if I draw more, I will assume that must affect fuel economy to some degree as this is a load on the car.

Does the Prius have an alternator like a standard car has? That was my other question.

Thank you in advance for all the help from you guys! When my dilemma is solved, I will post detailed information along with photos so others can duplicate.