Biggest thing about the seat is just getting one that fits right and also installing it correctly, so that you have the proper front-back, side to side weight per CRG's manual. A lot of people just slap them in, but I do get out the bathroom scales when mounting a seat and tweak the braces and amount of layback until I get the weights just right.
40mm axle should be perfect for that, again you don't have a 30HP TAG motor, and want to keep the kart "free" so you don't scrub off HP.