The big ticket items service wise are going to be reel, bedknife, bearings, and motor. If it fires right up, motor is probably fine. It really depends on what you're spending. If its a $500 or less mower, I wouldn't worry too much about being picky as you're likely going to need a lot of work. If you're at the $1000-$1500 range, then you can dig a little deeper expectation wise.

I bought a GM1600 fully serviced, sharpened, bearings changed, etc. from Prairie Turf Equipment for $1850 delivered. The thing with getting a used mower from an individual is once they have your money, they're gone, and I didn't know enough about what to look for, so my personal choice was just to go with a used equipment dealer that was trustworthy.

 

X2. I bought my 1996 GM 1000 from my Toro distributor for $150. It was being retired because the golf course was replacing its fleet and traded in old for new. Not long after it was mine, it got new belts, blades, belt covers, and the clip kit. All told, I spent enough to make it a $1000-1500 mower. Because most golf courses in Hawaii are Seashore Paspalum(they are regretting it), the golf course water system is either sea water or reclaimed water. Washing off the clippings with salty water rusted out the belt covers.