Phil--
Welcome to Cobraland. My advise to you is: Research.
I found Curt Scott's Guide to Cobra Replicas an excellent source to start with (see
www.cobracountry.com), as well as this forum (just watch out for the guys trying to convince you that a full-race Pinto motor is the only way to go ).
Finally, go out there and DRIVE some of these things. See what works for YOU. There is every conceivable permutation of Cobra out there, and the more you see the easier it is for you to define priorities of price, power, chassis, cosmetics, resale value, etc.
Heck, the shopping is nearly as much fun as owning (almost)!