I paid a $9,000 for a '94 Freightliner FLD 120 with under 650K miles on it and had it delivered to Arizona from Tennessee for a further $1,000. It already had been stretched and had a 24ft Grumman body installed.
One can pick up '90's tractors for anything from $5,000 to whatever you can afford.
Of course, they will nearly always have a stick trans and so you will have to drive the stick or allow for either a World automatic or an Eaton Autoshift to be installed. As my truck was the last year of the Detroit DDEC 2, the World or Eaton were not directly compatible and there would be quite a lot of wiring and reprogramming involved. The estimates I got from 2 different trans shops were around $23,000 including the core charge, labor, the whole 9 yards. Hence I went for the long established Allison HT 740 four speed auto at around $2,500 to $3,500 depending if you want a rebuilt one (not including any core charge, if they want one), then the rear end gears have to be changed to make up for the lack of an overdrive or you only make 60 mph flat out.
Frame stretches, I was quoted around $5,000 and up, the length was not too much difference as it was just more steel unless you needed another carrier bearing in the drive shaft.
You can find longer wheelbase tractors and with a 24ft body, a 265" stock wheelbase is a pretty good match.
Heavy duty levellers, I have seen some that were about $3,500, I don't think this included installation.
Box bodies, I was quoted from $2,500 to $3,500 depending upon condition, etc. The dealer in Phoenix I spoke with would not instal it as they didn't have the liability insurance. They would not even give you the "U" bolts. They would pick it up and put it on your chassis, but then you were on your own.