Why do you think he's selling it for $14k? In order to sell the plane reliably for more than that, it requires a minimum of $20k spent in the panel, minimum, or people will not buy it. It also has the E-225 and electric prop, this makes it hard to sell, although there is a market. Money wise one might be better off pulling this engine and prop off in good condition and sell them to someone who wants them, and salvage a low/midtime IO-470 and prop. Personally I would keep the 225 at this point as I think there is a MoGas STC for it.
Considering all the rest of the recent airframe work, I'd suspect there's some more corrosion to deal with as well, and probably another bladder going bad.
By the time you can 'flip' this plane, if all you're going to do to get it in annual and shine it up, it's a '$25k airplane tops if you want to get it sold quickly. If you have a good buyer for the prop and engine, you can maybe pick up a few more thousand off the engine swap equation if you do all the wrenching and already have a replacement located. You'll get at most 30¢ on the dollar on avionics upgrades, but you may never sell it again without.