hey scott, I ran an 8t on my 4300 and 4200 and 9t on the 3900 and all work perfectly for my flying. I actually put a 12t on the 4200 rcsmart just to see what it did and it caused some lazy tail control, even the 10t was too much.
As far as parts, most of what I get comes from the great Kong, but if I want something quik I use RC HELICOPTER AIRPLANE CAR WORLD out of Texas. They are fast and have everything. I must give the advice that I follow to e an honest person so I really suggest going o enjoyhobbies and ding the arf's. I recently started doing it that way and yes it does take longer to get the parts but you do save quite a bit of money that way. If all I were saving was 10 or 20 bucks then I would just throw my support locally, but when you can buy EVERYTHING you will need in the event of a crash and still have spares for 76 bucks to my door, I got to go that way. I have actually wrecked a bird bad enough to rebuild the arf instead of the wrecked bird and I still have the wreckage to salvage out with ebay parts.
http://www.enjoyhobbies.net/ Check my man Huang out and see what you think. I religiously follow ebay and just won another hdx300 brushless combo for .99c and 9.85 shipping. looks like a cp2 is in for an upgrade. That is where you find the real bargains, I also picked an eflite hh gyro for a blade I have, factory in the package for 28 bucks including shipping.
Also look at helidirect aka helirus. great guy and some killer deals. he is also stateside. I have lots more and if you ever have trouble getting something just give me a shout and I can probably come up with it for you.