What follows are my opinions...no empirical basis, no facts, just opinions...
1) Pick a mid-power kit that will teach you building techniques that can act as a stepping stone to high-power. My criticism of the Estes PSII kits is their use of rubber bands & the "tea-bag" method of connecting the shock cord to the body tube. Those are low-power building techniques that aren't really going to help with mid- to high-power.
2) With #1 in mind, you don't necessarily want to job to totally alien materials & glues right away. Save the fiberglass & epoxy stuff for high-power. You want this to be a gradual learning curve, not a steep slope up (or down...).
3) Evaluate how big your flying field is, and what motors you can put into it. You may have a small field, in which case you could still fly big rockets on a G (or even an H, potentially). No point in getting a smaller kit, only to have it sail away (but the Jolly Logic Chute release could be helpful with this).
So, with that it mind...I would recommend you focus on 2.6" to 4" diameter kits, and I'm pretty much going to echo everything everybody said above about Madcow &/or LOC kits. Those Mercury kits look cool, I've never bought any.
No one has mentioned any Binder Design kits, I'll post a link to the 3" kits below. These kits are fantastic in that pretty much everything you need to fly is included with the kit, and the instructions are incredibly detailed & helpful. The instructions even say to call or email the owner if you have questions at any point, and Mike Fisher is on these forums quite often, so, I highly recommend these to anybody looking to get a start in mid- to high- power and is looking to learn modeling techniques for these rockets.
http://binderdesign.com/store/page17.html