There isn't a lot of info here on GTRVs, but they are a GREAT product, too bad the original shop isn't making any more of them . Here is my experience having owned a 1999 GTRV V10, 2001 GTRV 7.3 longbody, and a 2006 SMB RB50:
• Cabinets are made of high quality plywood, not the pressed particle board SMB uses.
• Lower bed is a true queen side bed, SMB is only a double size.
• Upper bunk is wider than SMB
• Top profile is much lower and sleeker on the GTRV...great for stealth camping. However, the SMB T-shaped top definitely has more head space than the V-shaped GTRV top.
• Van upholstery quality is very similar, but poptop canvas is higher quality on the SMB.
• GTRVs have side popout window as standard, and the camper windows are larger.
• GTRV headliners all sag but they are relatively easy to replace yourself.
I've seen under a lot of Econolines and yours looks normal compared to all of them except the southwest...and even there they often have surface rust too. What I can see needs to be treated but nothing too serious.
$13-14K would be a steal...even at 15K firm I bet it will sell fast. I sold my 1999 V10 GTRV with similar mileage last year for $22,000. It was an E350 with the V10 so those do fetch more.
The 4.6 5.3 and V10 are from the same modular family of engines and should be good enough. Best of luck...let us know if you snag it.