I've owned a couple of R75/5s, they are beautiful machines and built to last about 300 years. You just have to be ok with the old tech, like drum brakes and a 4-speed trans that sucks on the highway. The /6 models were a big improvement in road performance but also a big step backwards in looks, with (modern at the time) gauges and big plastic turn signals, etc.
The prices have crept up to where $3500 seems about right for a nice rider that does not have any real red flag issues or needs, it should also run well and not have a bunch of stories. I'm a little concerned that it "runs rough" after a qualified mechanic has adjusted the valves and carbs. It should purr like a kitten. Those bikes are about as simple as it gets in the world of motorcycles. If they have good compression and carbs are not mucked-up, it should idle and rev really smoothly.
I wish that I had snapped some pics of the nice one I had in the mid-'90s, that was before the digital camera age when everyone took hundreds of pictures of their machinery, (including me now).
I paid $1300 for it outside of Minneapolis and it was in close to showroom condition with low miles. It had a big, goofy fairing on it that I removed to reveal a show quality bike. I sold it for double what I paid later, arghhh...
