Look at the suspension. Most fld-120 were road tractors, some air suspensions don't take well to the abuse and off road conditions that a lot of dumps go through. Look for cracks/ stresses in the leafs and their mounting points, and the air bag mounts. That said, a air ride is a lot better for your back, and if your just hauling for yourself it should be fine. There's a ton of converted road trucks out there running as dumps, I'm guilty of making one of them myself- I did use heavy frame, spring ride truck though.
Look for holes or worn spots in the bed, it will get more wavy and conform to the under frame, the more its been used. Look for cracks or stresses in the frame rail/ crossmembers, (typically happen between cab and tandems, close to the cab where the cylinder twists the frame). Look also at the rear pivot, you'll get a idea how much its been used by the wear in the rear pins.
See what the rear axle ratio is. If its a converted road truck with 3.55 rears, its not going to be easy to crawl around a jobsite, or spread rock as your unloading.
Good rubber is nice, but you can buy tires, frames are not so easy to buy/ replace. Engine, transmission, rears: you can tell a little by driving, and look for excessive leaks/blowby, but honestly any one of them could go at any time. I usually look at the condition of the rest of the truck, if they've not kept up with that, they probably haven't maintained the mechanical stuff either.
Do check the brake linings and drums. Its a wear item, I think the last truck I did was around 600 for drums and brake shoes (with maybe some extra parts). Its a wear/cost item like tires. If you get new, when buying the truck, for no extra cost, great, but if not, budget for replacing it, and deduct it from the value you place on the truck.
Dump trucks are all over the map in $ by me. There's a lot of $12-20,000, good usable trucks. Then there's a lot of $30-50,000 trucks that are just covered up with chrome, visors,big stacks, and lights. I realize some of them have spent $10,000 on chrome, but it doesn't haul any more dirt, or make any more $. I actually prefer a clean, straight, regular mack truck with steel wheels, than a all tricked out pete with a 6' tall shifter, and a chrome visor that blocks 1/2 the windshield. But that's just personal, everyone's got different tastes.
If your not real familiar with big trucks, find someone who is and take them with you when you go look.
I've been posting some pictures on here of lowboy pullers I'm looking at. I've bought and sold some trucks, but I haven't even touched on the endless combinations that are out there. Advice never hurts, and most of it is free.
