I'm a long-time fan of the L5450 - it is old-school over-built, one of Kub's best products ever in my book, a really nice robust machine. The Ever Clutch is almost always trouble-free but abusive slipping can warp the disks leading to
a replacement job. For a while I had 2 L5450s, one with a cab and the other an open-station with canopy. No real trouble with either - 1 has 1500 hrs, the other around 2k. I sold the non-cab for $18k a few years ago and it's still worth that. For this one the $12k asking price with the LA1150 loader may not be out of line, even with the experience it has. BTW the loader comes off ok - I do mine 2-3 times a year, best done on a level surface.
Check the valve cover vent tube for excess blow-by (smoke). The exhaust on these engines smoke a little normally until warmed up. Look at the 2 dipsticks (engine and trans) for water or other contamination, and inspect the 2-element air filter - if inner or outer element is broken or missing it's usually a deal breaker. You can do a quick check of hydraulic system strength - with engine at idle, rolling the bucket forward should lift the front of the tractor easily. If it doesn't or if it needs more rpm, the front pump may be worn ($$).
To check the hydraulic clutch, carefully observe if it tends to creep either forward or back idling in gear, with clutch pedal up, shuttle lever in neutral. Block one rear wheel and jack up the other. The owner shouldn't object unless there's something to hide. For the test put it in 2wd, idle the engine in 1st low gear with shuttle still in neutral. Shift the shuttle into either forward or reverse - lifted wheel will turn. Move the shuttle back to neutral - the wheel should come to a stop. If it doesn't, try stopping it by hand - if it persists in turning that would indicate clutch drag, potentially a spendy repair.
The L5450 was intended for heavy use and when they turn up in the used market they are rarely pretty - but they are a lot of tractor and below 5000 hours they're definitely worth repairing. Good luck - Dick B