Joe
You are about to begin a great adventure in fun flying. Unless things have changed since I ordered my kit 6 years ago:

1) You'll probably get a fuel shut off valve with the firewall forward and engine order. I ordered the shutoff valve early and now have an extra.
2) The cabin vent is an accessory. You'll want to order that with the FWF since that is when you get the "bonnet" and firewall.
3) The lexan turtle deck came with my basic build kit.

You'll like (8) the bubble doors - a little extra elbow room.
If you're ordering a Rotax the cabin heater kit works really well!
Oil fill door kit (14) came with my FWF cowling kit
I bought the cargo kit (15), but used my sewing skills to extend it more to the rear subject to w&b (really light stuff). Do this before skinning the plane.
I did order just about everything else, but I'm flying a trigear

Best wishes and have fun building.

BobRS
SS7, Rotax 914, Flying Phase I
Albuquerque, NM