Some excellent ideas here, thanks.
I've thought quite seriously about the conversion idea, but I suspect there'd be worms everywhere.
In particular I'd be worried about the tension on the neck, and about the pickup choice.
Trouble is, it wouldn't really be possible to do a cheap "recce". By the time you've sourced & fitted extra tuners, replaced the nut, found a suitable bridge, and anchored the extra strings in the body, there's a pretty substantial investment gone into a mere experiment.
On balance, buying a cheap bass that was designed and built as an 8 and then trying to improve it must make more sense.