Re: heavy riders
Im 6'2" at 260lb and my no name china girl pulls me along fine.
Some mod like the ones mentioned above along with port matching and a tuned exhaust will go a long way in giving you more power.
You also have to look at the terrain your gonna be riding in to get the proper gearing.
e.g. hilly area vs. flat ground riding
Another must have is a good strong wheel set. My first build had a cheepo single walled rims and dang did i put them out of true and pop multiple spokes in the first 30 min of riding lol