So, About six weeks ago I bought a 2014 800 EX with about 1250 miles on it for $5000 USD. It just had a new drive belt, winch and 8-ply tires. I drove it around for a few days and mechanically it seemed pretty good, but was prone to overheat. I removed the bodywork and found this machine had been rode hard and hung up wet.

Issues:
1) Had been completely submarined in heavy mud. It was everywhere an external power-washer couldn't get to. Had to remove all the body work and powerwash inside and out. Easily lost 50lbs of mud, half of that trapped over the belly skid plate. Elsewhere entire parts of the engine were entombed in dried mud.
2) Overheated easily - Radiators fins were plugged with mud. Also, replaced thermostat, flushed radiator.
3) Fan switch intermittent - Replaced fan switch (twice because I got the round connector initially and needed oval)
4) Brakes were very weak - Replaced all pads and master cylinder (front brakes weren't working at all), bled.
5) eBrake inop - Adjusted cable, replaced pads.
6) OEM skidplate destroyed - Had to cut it off, will need something after market, running without one now.
7) Driver side seatbelt locked up - Replaced.
Differential drain bolts seized - gave up trying to get them out, vacuumed out the gear oil best I could and replaced.

I stripped it down to the frame and cleaned it, changed all the fluids. Parts needed in order of expense:
1) Skid plate $200-$500 depending on OEM plastic or aftermarket metal -
2) Master Cylinder - $230
3) Seatbelt - $150
4) Brake Pads- $120
5) Fan Switch, Thermostat, fluids, fuel filter, air filter, extra hardware, plastic pop fasteners - $300

So far spent about $800 on parts (+ the skid plate eventually). Everything is working now, no overheating, good brakes, good power. So, I'm all in for about $6200 ($400 for tax/registration) + about 40 hours of work.

[2015 Z Force 800 53/Trail EPS]

Good...now get some use out of it KBB says it's worth about 6250.