well... you have so much to choose from. you can go with the reba or a fox f series. Just make sure that you don't get a fork that is too big and through the geometry off. the decide if you want sram or shimano shifter and rear derail. also if it is an m2 frame ill assume it's an older bike b/c now they have moved to an m5 frame. Saying that i don't know if it would be worth the extra money going with carbon fiber components, but that's just me. As for the seat post size i believe that it is a 30.9 you will need. most all specialized mtb have that size, but just go down to a bike shop and ask them to measure it for you. good luck man, it defiantly won't be cheaper but you will learn a ton!