Are you talking alfalfa type hay or brome/burmuda grass type hay? Where I live, alfalfa cuttings are 4-5 times a year and the other 1-2 cuttings/year. When I was younger, I ran a sickle mower for hours. Two, twenty-five acre alfalfa patches (9' mower bar) and each (25 acres) took about 8 hours mowing time. Around and around and around...... Then I would rake it - 7-8 hours. Then bale it with an old AC round baler - 6-7 hours (plus repairs). Then used a bale loader to put in on trucks and into the barn - 5-7 hours (3 people min.). Now double the above time for 50 acres. How bad do you need the hay? Consider having the hay done by someone and splitting the hay. In my area, the guy putting up the hay and baling it typically takes half for grass-type hay and less for alfalfa. Still have the 9' sickle mower which is in very good shape and sits inside except for occassionally mowing roadsides (steep ditches and the bar hangs over the edge.)
If still considering buying, then the major points of wear are the pivot points where sickle bar attaches to the drive mechanism after the PTO shaft. I used a NH mower which was PTO driven and then belt driven to the oscillating mechanism that drives the sickle bar. Check the wear of the bar. Individual bad sections can be replaced and riveted. A really bad bar can be entirely replaced. I have only use 3pt mounted mowers but many older pull types are more common. In my area a 3pt mounted mower in good to very good condition like mine will still bring $750-$1000 because people want them to mow roadsides, waterways etc. A 7-9' sickle mower is still much faster than a bushog rotary mower but won't take near the abuse.