I would also suggest inspecting all the moving joints, pins and bushings, for wear, and slop, , and or if they look like they have been greased often or not much during the yrs! old crusty grease means OLD and not greased often, bad sign, on care , lots of fresh grease means were greased more often! a good sign
I would check welds on arms and al over for what looks like NON OEM welds, folks that abuse machines , tend to break welds (all the more so on weaker spots, like lift arms, and likes) broken repaired welds, tends to show signs of harder or abused use!
I would check fluids, for a few things, one, if NEW fluids,good and bad, , as keep in mind the last many yrs we had a LOT of floods all over the USA, and that could mean a machine was under water, water in fluid, can be spotted, but harder once a person changes it
when machine get water in they typically get dirt with that water, and that means longer it runs after fresh fluids, the more wear and tear happens, at a faster rate
ain't no way to drain fluids and remove dirt that got in once water took it there, complete tear downs and cleaning are the only way to resolve flooded motors and likes!
now, NOT saying they were flooded, but just keep that in mind, pending where your located and the machines were!
visually inspect ALL hoses, hydraulic hoses are not cheap and they can add up to a bunch of $$ fast, and as stated, watch for smoke on exhaust, leaks,
run machine, lift front tires up,with bucket, and leave it there a bit to see how fast or slow it leaks down, or if it holds things up at all
check tires for dry rot, all the same size ? , look at filters(oil, fuel, hydraulic
lots of folks when replacing mark date on them as to when changed last,
but folks trying to scam buyers , tend to know this and stick new ones on trying to show new fluids been done ,(or they are honest and just changed them, goes both ways) but we do live in a world where a persons word is not as reliable as it used to be and a lot of scammers out there any more IMO