Glad you posted some photos, a friend has one of these in a 5'-0" width, and I'd never would have identified it if you did not post about your potential acquisition of one ! I changed blades on it last year for them, took a few photos etc. same model.
A used rotary cutter needs to be looked at carefully, as people tend to not be so nice to them.
Check the gearbox, see if it has oil, noticeable leaks etc. Many never seem to want to take the gearbox off the deck and replace worn seals, often times there can be a mix of oil/grease. There are soap bases and compounds that may not be compatible when mixed. Not sure how that effects internal parts on these long term or what if any concerns there are with one run like that, its a practice I don't subscribe to, if possible fix the seal and use the specified lube. NLGI 00 or similar/equivalent being less viscous, is often used to reduce or stop leaks. gearbox should be vented, seems they will build pressure and could rupture a seal.
Check to see if the gearbox is loose on the deck, wallowed out holes etc. from being loose.
Brush and safety guards, you should easily be able to make and or fit whatever should be in place or would provide dampening to any projectile like objects a blade could toss out, still use care when operating, avoid unknown areas or places where hazards exist, walk and inspect the area first to reduce the possibility. I know what they can do without guards, send a cantaloupe size rock up 50'-0" and over a hedge row of trees, went right up the backside of the 2010 JD I was on.
Output shaft to blade holder....if there is excess play, it could be an internal problem, bearing or similar, that would be of grave concern if so, finding parts for that particular gearbox or the cost of replacing it would be a consideration.
Heavy flaking and or thick scales of rust coming off, that may weaken the deck, and be a safety issue, surface rust, not so heavy pitting, should not be an issue. These seem to be built heavy. The one I worked on did not have a stump pan, just the steel cross piece/blade holder. I thought the deck was well built, maybe its just being of older manufacture, it has thicker steel.
Blade condition is important to cutting efficiency and vibration, they should be equal weight and or balanced, if heavily deformed or gouged, they either need repair, or ideally to be safe.... just replace them. This mower has some short stubby, really heavy blades, well the one I worked on did. Its a lot heavier built than my SE-6 Rhino.
Additionally, these need to have blades in good condition and be properly adjusted to the tractor, typically, level side to side and the front set lower than the back by an inch or so. I found books for this mower on ebay, click below for those listings.
PTO safety guard should be installed, though who ever goes near one of these while its running, well besides whomever is in the seat ? Grade #2 bolt in the input shaft to the pto shaft is there to shear, don't use anything else and replace whats in there with a known grade #2. If it does not shear, all of that shock load is going to be absorbed elsewhere and can cause damage. Friend runs his with a JD 2020, and someone had the wrong grade in there, they hit things over time and it destroyed the slide coupler assembly on the PTO shaft, way down in the cast housing, which then required a complete tear down of the back end of the tractor. This was a 2 stage clutch. Grade 2 bolts are a lot less expensive. Some blade holders use left hand thread shear type shoulder bolts, my Rhino has them and they work well, when you hit something, no damage to anything, just the shears off bolts, and all the parts remain under the deck, no shrapnel effect.
Used mower of unknown condition, look it over good, if things check out, that's a decent price, once gone through and everything taken care of, it can be a worthy rotary cutter. I did that with the old SE-6 Rhino, 7-10 acres a year, going on 13 years. Keep my blades sharp or replaced when needed, and change the lube in the gear box.
M F 59 mnauals