Depending on your actual skill level, been racing other forms of moto or not, I would suggest that you start in a (c) age class at least for the first race or two so you can judge for yourself how you stack up against the competition.
As far as fuel tanks go, the stock is adequate most of the time in enduros,but you could possibly benefit from a larger aftermarket tank in h/scrambles or cross country races for fewer fuel stops. The big deal is just get out there and have fun the first few times, I think you'll be amazed at just how good and kind enduro folks are!