I was looking at kits myself a while back and found that they were missing a couple of pieces, and generally locked you in to a "proprietary" PCM that was "tuned" by the place selling the kit. Those tuned PCMs have been known not to be the best, to say the least. (Stay away from ZZP tuned PCMs for example. Go with a DHP PCM.) I opted to read the "Safely Modding..." thread and have gone from there.
Here is a link to make it easier on you...
Safely Modding Your 3800 Powered Car