My last couple of forays have been into the middle territory - turd polishing, I think it is called Except not all have been turds.
I customised a Westone Thunder Jet, bought for £77 on eBay, with a pup upgrade, high mass bridge and a J. East P-retro. The body and neck were of good enough quality to justify this and I have have a unique and fun lightweight bass that sounds the business. When it comes to move it on I can revert it back to stock and sell the extra bits without losing too much.
My second turd was a Fender Precision Lyte, bought unseen for £300. When it arrived it was a dog - really badly refinished and the electrics didn't work properly. I couldn't live with it as it was but would have taken a massive hit trying to resell it. Instead of cutting my losses I decided to treat it as a teaching aid and completely renovated it - refinished the body from orange peel red to a decent gloss black, took the headstock back to natural from red. Shielded all the cavities and it now sports a custom 3 band eq from J-East bought second hand from sliddx. I have learnt an immense amount from doing this, have bonded immeasurably with the instrument and thoroughly enjoyed the process, too. I'll never get my cash back on this but it is now pretty much my perfect bass.
I think as long as the start instrument is of decent enough quality this is worth exploring.