I can't advise you on which way to go as that is a personal decision.
$4K for a "pig-in-a-poke", "rebuilt" 807 is way too much. Not that you can't easily spend that much if you are doing a performance engine but there is, as you say, too much missing and there's no info on what was actually done. Someone on the yahoo list spoke with the seller and they were just as non-committal in person as in their ebay description.
I went the Renault hemi route as I like to stick (mostly) to period mods. I bought a complete, running, early Renault 17 Gordini for $1000cdn. The engine was a mid-production 807-13 (FI, 9CR, press fit-pins, large valves) with an early 395 transaxle (closer ratios than a 365). Swapped in 10.25CR pistons and liners with billet rods (floating pins), had the cam reground to fast road spec and fit twin 40 Dells. Machining, balancing and parts totals are probably nudging much closer to $4KUS than I would EVER admit to my wife. Carbs alone are filled with gold as it's expensive buying brass for four bores each time you tweak the jetting.
