Seems a bit high priced, I bought a 99 F350 4x4 7.3L with 47,000 miles for $15k and I know that might have been a bit high when I bought it last fall.
It is replacing my 99 GMC 3500HD Flatbed dump with 3' sides that I built, works great for loading soil or stone, yes the tailgate can be a pain at times but it works just have to pry at it if your load is against the tailgate. The sides can come off so you can load from the side of the truck which is handy at times too.
See if you can get that price down on that truck and then work on the sides using 2x4's and plywood. Then paint it.
1999 F350 PSDiesel 9' Dump bed 8'6" Plow
2005 Chevy 2500HD 6.0 Gas 8' Western Plow & salter
2002 F250 PSDiesel 4x4 8' Western Plow & Salter
2003 8.5x16' Great Lakes Cargo Enclosed Trailer
2005 6x20' BNM tandem 12,000lbs trailer
2011 Scag 61" Turf Tiger 29 LCKawi DFI
2005 Scag 61" Turf Tiger 29 LCKawi DFI
2004 Scag 72" Turf Tiger 29 LCKawi DFI
2001 Scag 52" Turf Tiger 23 LCKawi
2002 Scag 52" SWZ 17 Kawi
2003 JD 250 Series II 64hp