79lre

Hi all, I'm newbie to the classic Oldsmobile thing here and not an owner. However, I did find a 68 Olds 442 sitting in a guys yard/concrete driveway for sale. Here's the description both getting a quick glance in person and talking over the phone.
68 442 auto on the column; bench seat ripped horribly on driver's side, 400 4bbl with TH400; engine is suppose to be #s matching. I could care less; positrac unsure of gears but, thinks 3.42 maybe? had rear defrost; trunk release vacuum op; states prior owner let sit in leaky barn for 17 years so trunk pan is rusted good not sure how bad as I had not looked at that yet. bottom front of both fenders rusted out at very bottom; surface on bumpers; front of hood surface rust; claims 104,xxx miles and states runs and drives. Does not smoke or excessive use of oil per owner; states would need distributor cap. Guy is asking $5500.
I think the car is more along the lines of a $3500-$4500 myself but, I could be wrong. Now this leads into several other questions; how much am I looking at replacing the fenders; reproduction ones? The next thing I am a current Mopar owner looking to get out of the expensive Mopar world and get into a cheaper musclecar. I'm just curious on how expensive parts are for these cars. I'm not really interested in the trim because my intent with the car would be to slap on some fenders do a little tune up and spray bomb it with black primer (so I don't have to worry about some person scratching a expensive paint job a child did to my Challenger.) and get in drive/enjoy it.
Now with these cars what type of gas mileage do they get and how is the performance on them? I currently have a 79 Lil Red Express truck that was getting abount 13 mpg on the highway and performance of 15.5 in 1/4 mile with a 360 small block. Thank you in advance.