it is really a nice thing to buy a car and have everything already be working and everything looking nice, don't sacrifice quality for boost, it really is not worth it.
As for that paint on the roof, that really is not that bad now that I look at it again, my roof looks EXACTLY like that when I don't wax it for a long time. The scratches, yeah I have em too, really don't know where they came from but they are there, but they sort of go away with about 4 coats of some really good wax, so don't wory about that paint thing. For the bumper covers and trim and such, well that is up to you, if you feel like doing it, well its your wallet. And for the interior...personally I would walk because the results of sitting water in a car is definitely something you don't want to deal with. Wiring, Connectors, Floors, they all corrode and you will be footin the bill to fix it all.
As for the engine, well, even my engine looks like that because i'm really not the kind of person that really gives a dang to go and clean it. Infact i think mine looks worse, don't worry about the engine, although it does look as if someone left the hood open in a rain storm or something. The current owner of that car sounds like a real neglector. I'll bet hes the kinda person that never changed the oil or only changed it when he felt like it or something. But if you feel like a challenge, go ahead, but I say WALK, and keep looking.