Rollinthunder:
I'll share with you my experience buying a used truck.
I purchased a 1998 FL70 with 350,000 miles on the CAT 3126 engine with a 6 speed Eaton/Fuller trans. This truck has a 70" sleeper and a 24 foot box w/roll up door and good rubber. The truck was a repo and I depended on some help from a mechanic/family member to help me check it out. Mercedes Finance wanted over $20,000+ for the truck and I started bidding at $14,000. I negotiated hard and believe that I got this truck for $15,000 because Mercedes has more repo's that they want to be stuck with. (Large Freightliner dealers have repo's on the back lots. They get storage fees but not much else if you buy direct from Mercedes.) I have put 50,000 miles on the truck and it uses less than 1/2 gallon of oil between changes. It has no fluid leaks.
I will give the estimated costs for repairs done to the truck, much of which (w/family help) was done in my shop. Replaced trans linkage, new clutch, replaced exhaust manifold seal, new radiator; new brakes and chambers, new computer & injectors, new radiator, 3 new batteries etc. This is about $8,000 to $9,000 worth of work done on the truck in one year. This puts the truck in that $24,000 range you are considering. (I also threw an $11,000 Maxon 4,000 lbs. removeable lift on the rear.) Although rated higher, I tag this truck for 26,000 lbs. Because I drive from Michigan to east coast and mid-atlantic states I keep it under 26,000 so that I can get up the mountains in PA and NY. If I ran this at 34,000 lbs., I am sure my repairs on the truck would be higher.
The point of all this is, if you buy a $24,000 truck (or less) you better have the cash reserves to keep it operating. For $24M you will probably end up with an FL70 and you should expect high maintenance costs. Try to buy from a driver that "must sell" but that can provide maintenance records. My CAT 3126 runs fine at 400,000 miles (not a lot of power), but I do not expect to see 600,000 with it. The replacement cost (cannot rebuild) will be about $14,000.
Good Luck and Drive Safe