M818 may not be good
As cheap as it seems it would be with less than 20 grand into the whole deal..... there are alot of things that will not make this a cost effective choice for civilian flatbed trucking. Check internet sites such as truckpaper.com and local used truck places for the same things your into. Ebay is also a good place to get an idea of what your after. If you are as mechanically inclined as to be able to deal with the issues that will arise involving service to a military set up..... you are certainly just as capable to deal with that if its a KW, Pete, Western Star, Freightliner.... etc.
Some things that will make a difference:
Prep for D.O.T. inspection.... military will need mods to bumpers for the M818
Comfort for long term runs.... no sleeper means spending money on motels
Freightliner FLD 120 series are pretty much the same as M915A2s,3s,4s
Parts.... can't go to the shop with a -14&P and look for a part
Insurance
There is no doubt that a chromed out 5ton would be cool as hell but a full chrome Western Star is just as cool
Financing.... banks dont know military vehicles
20 grand is not unthinkable to start a rig up with and 15 grand is possible but do not invest so much that you cannot "afford" to lose. Being an Owner Operator..... It's a bit like motorcycle riding, dont buy a bike you cant total.
Check out the OOIDA website for info, lots of good info and if you do the membership for like 40 bucks they will send all sorts of even better info.
Size of deck is also an issue and if you are using a lowboy style vs. freigtht style it is something to look into
Good luck!!!!