Bikes have been standardized for years. So, what you need is a mix and match of the best products you want to purchase for your bike.
You can run pretty much any 20 inch tire on the market. Check www.danscomp.com
for a list of product out there.
Grips and standard (one size only really), and personal preference.
Brakes come in 3 varieties - Caliper (bolt in the center) U-Brakes (mounted on welded on posts on the frame) or Cantilever (also mounted on welded on posts on the frame). You may have to ask at a shop what type of brakes you have if you don't know. Once again, there are many different types of brakes of varying quality and cost.
ORIGINAL equipment is a big mistake. Unlike your car which has 10,000 parts specifically made for it, your bike is built around certain standards which have been established within the industry. One of the big changes is that your bike probably uses a 1 inch head tube while all newer bikes use 1 1/8 inch head tubes. The result is that if your forks have problems you may have a really difficult time finding quality replacement forks. BUT, you can find plenty of so-so replacement forks, it's not the end of the world.
is perhaps your best resource on parts and pricing for your bike to get an idea of how much stuff will cost. I am not sure of what other information you may be looking for. I would go through your bike and clean it up first, then figure out what you want to replace, then ask about specifics of what you are looking to do. Your local bike shop can help you out, but you will probably pay 50% more PLUS to have this service from them. But, it is up to you on that.