Well, I deal on a regular basis with a converted Motorola Mitrek on 70CM, 2 GE MastrIIs on 2M, a HiPro on 2M, and a HiPro on 70CM. I have also worked with a Kenwood TKR-750 in the past, as well as some Icom and Alinco mobiles.
The Mitrek is an easy conversion project. It also has turned out to be the the repeater that gives me the LEAST of problems, and actually does 40W out (fan on heatsink). I have actually built one of these with the tone board and controller contained inside the case, and a 50W mobile duplexer that worked well. (It was a 16W output radio). I normally use the speaker and control head so it has a "local" control system, using switches to break the mic and speaker connections.
The GE MastrIIs are a learning project for me. I've had problems with TX exciters and power amps.
The HiPros are a pretty good machine, and work well as exciters. If you have a good location, the power amp add ons are good. Everything is standard discrete components, no surface mount. They also come in a nice rack mount case. On ours, we run external MOBILE amps with big heat sinks and fan banks controlled by a timer circuit keyed by the PTT. And, they HAVE NOT failed.
(NOTE: the fan timer I use is a CK1614 Multi Mode Timer from Carl's Electronics. It is a kit, and I have never had one fail.)
The Kenwood TKR-750 was OK, but it does require very careful tuning which is all done by software.
As far as using mobiles for a repeater, I see no problem doing it, with one exception... they were not designed to run at full power key down for a long time. That said, run them on low power as an exciter into an external amp. The plus, is that you have a repeater that is fully frequency and tone agile. I would prefer the Alincos, simply because that almost all of the case is heat sink, and they have the rear panel interfacing port, with everything you need brought out the back. Don't forget the fans!
Main thing to remember on building a repeater system is good location, good antenna, good heliax, good duplexer, and good sensitivity in the RX.
NA4IT