What do you guys think a 4cy to 5.0..

I have a 91 GT,its abouta 3 out of 10 right now. I mean it runs good but it looks like crap and chatters down the road.Im restoring it. I wanted to geta 4cy as a donor.And as you all know for some reason 4cyl are usually cheap,clean,and unmolested. MY 5.0 has frame damage and doesnt ride to good my steering wheel shakes overall its normal for an old stang that was beat up on.it has all sorts of inerior rigs like wooden door panels,bootleg window switches..the dash lights broken speedo off(stock gears!!??anyways alot of little stuff that is fixable but never gonna work like new...anyways i can probably pick up a clean dependable 4cy to drive now save on gas and insurance etc...Anyways my question is should i rape the 5.0 or the 4cy..either way the amount of work is the same.I want to pull the gt's engine and rebuild it i can do that while im driving the 4banger. have it ready to swap. rebuild my t-5,and rear end etc... and drop it all into my 4 cycly(when and if)....what do you guys think...
ps how much do you think i can talk this guy down to if i pay all cash??
http://www.autotrader.com/fyc/vdp.j..._year=1993&start_year=1981&lang=en&cardist=13