The far bigger concern with a '07 Silverado Flex Fuel truck would be determining if it had an oil-burning AFM engine. You'd be out of GM's powertrain warranty, so I'd insist on at least a 3 year/36K mile extended warranty paid by the dealer, and make damned sure it covered the engine.
Like you said, there are way too many other used trucks out there to go wading into that thicket.
FC
