Get a couple of spare arms with the quad. I haven't used any of mine yet, but better safe than sorry!
Lipo - get something around 3000mah and 3cell.
If it's available, get an integrated power distribution board for your frame - much neater and therefore easier to install extra's in the long run. If you don't get one (and don't want to solder motors to esc's) then you'll need ways of joining them. Most motors and esc's seem to come with 3mm bullet connectors installed, and the other ends of my esc's came with either XT60's (on the turnigy TI) and 3.5mm bullets (on the afro's). Originally I used this PDB:
http://www.hobbyking.com/hobbyking/s...dProduct=23140 which worked fine, but have since changed to integrated.
You'll need these:
http://www.hobbyking.com/hobbyking/s...dProduct=21758 to join KK2 and receiver.
Props - 1045 SF (slowfly) for a 450, athough you can use 9", or 8045 on 4cell. You need 2x CW and 2xCCW. Buy spares!
My KK2 is mounted on standoff's (DO NOT USE METAL TO MOUNT THE KK2!) but you can use anti-vibration stickypads if you like.
These are my motors:
http://www.hobbyking.com/hobbyking/s...dProduct=12921 and I really like them. I've since ordered some sunnysky ones, but intend to use these in a tri (along with my old esc's)
Don't worry about a USBasp flashing tool (for the KK2) - you can borrow mine whenever you need to. I'm about to go onto 1.6++, so after that I won't need it for a while.
Velcro - to attach your battery's to the quad. I use these:
http://www.hobbyking.com/hobbyking/s...dProduct=11736 as well as velcro.
Prop balancer:
http://www.hobbyking.com/hobbyking/s...dProduct=20982
I think that's most of what I have now. DO NOT order anything from hobbyking without getting in touch with me, or Andy, first - we can provide you with "buddycodes" that will give you discount (and us store credit!)
HTH