After giving it some thought, I would have to say the two stand outs in my mind would be a 123 240D or any Volvo 240 series with a 4-cyl. The only thing tough to repair on a 4-cyl Volvo of that series is the S shaped water hose on the back of the engine.

The 740 Volvos are pretty easy to fix, too, which is a good thing because they sure seem to break down a lot.

The 115 Mercedes were just a little too tight in the engine bay. Parts are cheap but you have to tear half the front end out just to replace the water pump.

An Audi 5000, if you could find one, would be easy to work on unless you have an electrical problem or need to replace the heater core. The book on the heater core is six hours and you will need everyone of them.

The car with the most room is probably the Geo Metro since you don't really know if the 3-cyl you are looking at is the real engine since it is so small. But everything is easy to get to!