Now I don't have experience, but from seeing some Muay Thai fights, if you don't have the reach you need to have the speed. Not just punching/kicking speed, but you need to be able to move around. You'll also need conditioning so that you'll be able to constantly move around for the whole fight.
As for moving up in weight class, you can try it and see how you like it and if you don't, then you can move back down. But really, at 145 lbs, there aren't going to be many tall guys in the weight class, so it won't be as big a problem. The only problem is when you train with somebody not in your weight class.