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.
https://www.ebay.com/itm/292236490598?ul_noapp=true
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.