I know it may sound expensive in the begining...but, go for the SE. It will save you a bunch of cash in the end. I bought a 450S as my first Trex..fell in love with it and crashed it alot. I upgraded all the parts that kept breaking and spent way more than an SE would have cost if I had just bought it first. Plus, the metal parts have less slop and perform way better.

As for CDE OR HDE:

CDE has direct linkage from the servo to the swash plate= very fast slop free control
HDE uses levers conected to the servo then to linkage to the swash= more possibilty for slop and a little slower.

If you get the DX7, You'll want CCPM CDE. and set your expo to 30% on aileron and elevator to make the heli less skitish

I highly recommend 325 wood blades, then won't do as much damage to the heli in a crash and are pretty cheap $12 a pair. The extra length gives a more stable hover.