You do not mention if you intend to use a cross-band or in-band repeater. A cross band repeater would be from your regular VHF mobile to say a UHF radio. Technically much easier to implement in a vehicle, but the drawback is you'll have to scrape up some UHF portables to use the dang thing.
I suspect you want an in-band vehicle repeater to be able to use the handhelds you already have. Doable, but not for the faint of heart and requires at least the blessing of a qualified radio tech, let alone a full checkout to make sure the system works as advertised.
You say you have something that was pulled from service. What is this and what parts and pieces did this unit comprise of? Did it work for you before or did it have problems with it.
The things to consider on an in-band vehicle repeater are:
The extender frequency you intend to use. It needs to at a minimum of 2 mHz away from any frequency you
anticipate the regular mobile radio would use. 5 mHz or more is a lot better. If you do not have a frequency
you are licensed for that would fit, you'll have to apply for one. The rest of the county channels you have
would need no modification to the license.
You mentioned adding an amp to the extender radio. NO! Most vehicle repeaters run in the 0.5 to 2.0 watt
range. Your concern is how to crank a radio down to run that low. For on scene communications, 2 watts
You will require special filtering added to the antenna line to separate one radios transmit from the others
receive. Do not try and cheap out by omitting these, they are required. Also antenna placement on the
rig plays a big role in how well things will work.
I have installed my share of these, and always used ready made commercial gear from Pyramid Communications. If anything, check out their website before planning anything. They provide excellent information.
Pyramid's SVR - 200 - Pyramid Communications
In-Band Filters - Pyramid Communications