As mentioned things to look for are leadscrews, table/way wear, knee wear, spindle noise, and smooth operation of the quill. I would look for a machine with a 1-1/2 HP or larger motor, and a 9" x 42" table. Tables come in several sizes, I believe from 36" to 49". Some of the earlier machines are as little as 1/2 HP. While the do work, the larger HP machines, in my opinion are more desirable. The 42" table is the most popular size, and gives a good work envelope.
To check for table/way wear I would loosen the table locks and grab the table at one end. Then attempt to jiggle it in both the X and Y directions. There shouldn't be any noticeable movement. If it has a DRO watch to see if the values on the screen change. A couple tenths would be acceptable, but not much more. I would then tighten the locks and attempt to move the table. If the table moves easily the locks and possibly the ways are worn.
To check the leadscrews in the X, Y, and Z directions move the crank in one direction until you see the table move. Set the dial at 0 then turn the crank in the opposite direction. A machine in good condition should have less than .010" registering on the dial before the table begins to move. They come new from the factory with about .003" play. Up to .030" could be tolerated for some period of time, but it would indicate the leadscrew or nuts are worn, and will eventually need to be replaced. Nuts aren't that expensive, but a leadscrew can cost upwards of $300.00 ways. The table should move smoothly the entire length of travel in the X and Y directions. Tightness at the ends indicates possibly worn ways or leadscrew.
Run the head through the entire RPM range. There should be no abnormal noise or vibration at any speed. Also be sure to move the head between the high and low range. It should slide easily. If it doesn't it's a sign of poor lubrication. Also be sure to check the spindle brake. It should hold the spindle steady when loosening or tightening the drawbar.
As for the choice of heads I prefer the variable speed. If you intend to drill and tap a lot it's far easier to change speeds with a variable speed head than having to reposition the belt. These heads are a bit more difficult to rebuild if needed, but in my opinion the difficulty is far outweighed by the convenience of easily switching speeds. Even if you do find the head needs to be rebuilt it's a once in a lifetime project for a hobbyist. My machine was built in 1972, used in a high school for nearly 20 years, put in storage for another 10 and has spent the last 17 years in my shop. The head is still in good shape.
I would make sure the ram a will move forward and back on the ways, the head can be nodded, and the turret can be swiveled left to right. The head should also tilt left to right. Be careful when loosening the lock nuts. Make sure there is still enough tension to put pressure on the head. If the worm gear is damaged or disconnected the head could swing down quickly causing damage or injury.
Make sure the quill lock, down feed handwheel, and auto down feed are operational. Insert a collet in the spindle to determine if the alignment setscrew is present, and if it is damaged. Many people remove the setscrew, which isn't a big deal. However if the collet won't fully seat it's possible the setscrew or the drawbar are damaged.
If possible I would also attempt to tram the head in several places on the table. It should be the same throughout the length of travel. Different reading would indicate a possible loose gib, bent table, or wear on the table or ways.
One last thing. Be aware of new paint, and a claim that the machine has been "rebuilt". Many times a rebuild consists of a wash job and paint. If the machine was truly rebuilt ask to see the receipts for the parts. The one time I did buy a "rebuilt" machine (in this case a lathe) the owner was happy to show me the parts list, receipts, and repairs needed.
I'm sure I've forgotten a few things. Hopefully someone else will chime in with the things, I've missed.
