yeah swapping shells is more work than first thought. It may sound stupid but you have got a lot of bits to swap over.. well everything lol! You also have to remember what order it came out in and put it back in the reverse order.
You also have to take into consideration this; every car resonates at a different frequency and all the parts of the car are in harmony with one another.. kind of like comfy old shoes. If you then change a massive part of where the frequencies resonate too then it may feel like a different car. I know this sounds stupid but it is the law of physics and 100% true. You may find that you get some annoying rattles that you cant locate and it is simply the parts getting used to each other again.
My advise is that you stick to the car you already got, depending on whether it is really bad and just get it repaired and then sprayed. I know of a very good and cheap sprayers in banbury called sg customs. I was recommended them by a kind chap on here and they quoted me a mere 500 quid to have my car fully blown over. I was told that they also have done a few fast ford cover cars so there reputation speaks for itself. I haven't got the number to hand but just google sg customs and they will come up with a mobile number... give it a call and tell him someone on here sent you, he should give you a very good price.
Say the spray costs you a grand, the panel a few quid the welding maybe 100 (if you are anywhere near oxford i have a very good friend who will give you the cheapest price on this planet guarunteed, he works for 10 quid an hour and is very good indeed. So that brings the grand total up to around or just under the 1200 quid mark. As for doors, there are loads on here when people break up there cars, shouldn't cost you anymore than 20 quid... pocket the change and know that you have a nice car!