Warrenoie,
Don't let the computer scare you too much, all they care about it being plugged in to their harness and getting power and ground. If you go too new on your donor car (like 2002+) some (but definitely not all) computers will require a few more things to run like body control modules and key transponder rings.
The very best thing you can do though, to make your life easier, is to buy a complete donor car (can be wrecked). That way you know for sure you have everything you need. A lot of times you can get a complete car with a perfectly acceptable motor/tranny for anywhere from a few hundred dollars to a couple thousand. Check your local car auctions, insurance holding lots, wrecker lots, local paper and craigslist and local auto salvage yards.
Instrument Cluster and Speedometer Repair