MOST pitman arm style mowers don't like being used with the cutterbar at anything other than 90 degrees (parallel with level ground)
Look for a mower simular to New Holland 451, Ford 515, MF #41, ect. I'm "old school" (read CHEAP) with my hayin' equipment. I'm STILL using a sicklebar to this day. I've tried 'em all. Best by far is the NH451. (NH450 ALMOST as good, but slightly weaker frame)
With a "wobble drive" (ie NO pitman arm) make sure there isn't much, if ANY slack in the drive. With the slightest movement of the drive pulley, you should get equal movement of the cutter bar.
Sections and gaurds can be replaced if worn. Generally, most used mowers will need new sections, probably new gaurds too. A good indication of over-all use (wear) is the skid shoes on each end of the cutterbar. If they're worn excessively, good chance the mower has a gazillion acres behind it. Not automatically a GIANT RED FLAG, but a sign to look further for signs of extreme wear.
Look for welds (repairs) on the frame itself. When using a sicklebar for hay mowing, often they're used at a much higher speed than the "non-sicklebar mower user" would term "normal". The frames can take a beating. Mis-alignment problems are frequent with mowers that have been bent, broke, and battered.
