I agree. $900 is too much. I bought a running truck for $650, although the body wasn't as good as the one you described. And I bought a couple out of fields that weren't running for $200 and $250.
You don't know what else is wrong with the truck, so I'd start at $500 and go up from there, being happy if I got it for $650 to $700.
And I say "happy" 'cause the body and bed, plus maybe the interior, are hard to find any more in that shape. Go for it! Use the body and bed and part the rest out on here.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI