It is not so much what spur is being used but what diff mounts are being used - If buggy then yes get the CR buggy mount.
A123 are nice but heavier/bigger and only 4600mah in 2s config - You wound need a min of 2s5p.
Pinion size - just use this site
http://scriptasylum.com/rc_speed/ and gear for 35-40mph
RX pack no - the ESC has a bec built in
Brake servo - Not really required motor breaks work great
30 mins is V tough - 15 is pretty easy though