IMO, I wouldn't throw any major money at the bike. It's a nice piece as is. If you want more gears and suspension buy a new bike. It's going to cost way more to upgrade what you already have. If the shifter sticks try flushing it out with WD40 or brake cleaning fluid. It should be as good as new. The supply of 1" suspension forks is very limited. Even more limited with disc brake tabs. If you do manage to find one you have to have a disc/V-brake lever for the front and canti for the rear. That is unless you go V-brake in the rear, more money. Not to mention the cost of new disc hubs. Are you going to pay a shop to build new wheels or do them yourself? If you sit down and really plan this out you'll find it won't be cheap.
A thorough overhaul of you current bike just might surprise you. Good luck