I've also thought of building my own car. I have a milling machine and lathe, and access to a cnc router. However, buying a kit was much easier. Type in paypal password and address, hit enter, and wait a few days for it to arrive. A few hours of assembly time, and you have a car! Granted, designing and building your own would be quite rewarding.
There are some things that I don't like about most kits, though, so I think it would be best to buy a kit, and then change the things that you don't like about them.
Me, I don't like the use of the c-hubs and single upper control arm. I'd rather see a true double a-frame setup like the front of the F1 cars use. I think it would be more sturdy, and require less parts. Using tie-rods in the rear to change rear toe would be nice, too. Being able to adjust the rear toe curve would then be possible.
As for an affordable first kit, I did a fair bit of reading about that when I was looking fgor my first car a few weeks ago. I was thinking of getting a Sakura Xi sport, but ended up getting a Spec-R R1 Pro Spec. $240 from RCMart. I'm quite happy with the kit, with the exception of the ball cups. Buy some $3 tamiya ball cups for it. I think the Sakura's might be a tad more fragile, and the upgrade path for the Xi sport was confusing, since it uses some parts from the Ultimate, some from the Zero S, some from here, some from there, etc.