A couple of points:
Deal with someone that has a great (not just good) reputation.
Builders come and go, find one that's been around and WILL be around.
Down the road you will need various "parts", that may not be in the "kit". Those parts will be easier to find through a number of outlets if you stay with a well recognized manufacturer.
The replica market varies widely in it's approach to "originality" vs your "re-interpretation" of the car. 427 is a good call, put's you on a path toward "originality", but to what degree? 5 speed is good, but not original. Go with a 5 speed BUT try to get one with the reverse shifter so you keep the "look" original. Another option and save a few dollars, go with a 428. Many owner/builders opt for a small block, which is "correct" ONLY in the 289 FIA body style replica. But the small block works well and saves cash!
9" (or a newer 8.8) saves some money over IRS and it is more "original". It generally has a smoother ride. MORE expensive and difficult to build how ever.
There are three KEY items to retaining the "correct" look and feel. I would NOT compromise on these.
The right BODY shape or close as possible to it, they vary a LOT. The WHEELS! Custom wheels might look "cool" to you, but you may be the only one that thinks so!
The trans SHIFTER, got to be a "reverse" linkage setup.
After those key items I would consider the "look" of the interior. The dash, cutsom wood or custom stainless steel can look "great", but it's a dead give a way the car is a "re-interpretation" of the original. The gauges AND where the gauges are placed.
The roll bar has a HUGE impact on the "look". Safety says get a full width bar that covers driver and passenger. Some go with two hoops, one per seat. Looks more original with a single hoop, tough call here on what to do!
Generally speaking the closer to "original" the more the cost.
You don't have to be 100% dedicated to original "look and feel" but it's important to KNOW what that is/was and where you CHOOSE to be different.
For instance, BDR uses BMW suspension, it's good stuff, hi tech, but hardly original. Then again, you can't "see" those parts! I'm not sure that make will accept a 427 however. With the BMW trans you probably cannot get the right look with the shifter.
Hunter, out of Florida, uses Vette suspension, how does this impact your engine choice is the kind of question to ask. How does the over all "look and feel" compare to other makes?
FFR is VERY common, been around a LONG time, WILL be here down the road. The body is "close", but not all would agree "close enough". Typical FFR runs a small block, but SOME do run a 427.
Classic Roadster is a little wider and longer, I had a replica very similiar to the C.R. and like the added room and comfort. Eventually I wanted to get more "real" and went with an ERA for more originality. But there is a HUGE price difference between the two.
Curt Scott's, "The Complete Guide to Cobra Replicas" (4th edition) is a must read for a newbie. You can find it here, at the "Cobra Book Store" link on Curts site.
http://www.cobracountry.com/home2.html
Good hunting!