I have SoU and I will be getting HotU eventually. Im gonna start out with a fighter to lvl. 8 and then go over to weapon master. Will I be able to use all teh features of HotU directly after I install it, even with a charachter I started before installing HotU?

I don't know if I understand your question correctly but you can't use anything from HotU before you have it installed. Once you have it installed you can normally use any SoU and normal NWN characters. IIRC weapon master is a HotU class so you can't use it till you've got HotU. I think you get those new options immediately after you install HotU since you'll be playing with the HotU disk since then, not 100% sure but at least 99%