So I debated on where to put this, I'm new, the topic fits into several categories, I didn't find what I wanted in the search, and so here it is. (I'll try to use the proper terms, but bear with me please)
I enjoy trail riding, have been inactive for a few years, and so have been looking for a way to get the heart pumping again. I'm a big guy (6'4") and came across an old Raleigh mountain bike with a 19" frame at goodwill. It's a little on the small side (I want a 21"), but at $25, I though it will get me going and if I enjoy it as much as I remember I can buy a new bike down the road. The shifters don't work though (i've 'engineered' them to at least cycle through 6/7 of the cassettes but I can't change my cranks) and the chain is jumping off my largest crank (some of the teeth are broken).
For now, I'm using the bike as a hybrid on streets and sidewalks, but as I get going I'd like to take it on some trails.
My current musing of a plan is to replace the cranks, shifters, cables, and tires... then later the de-railers and cassette. When I get some money saved and find it's worth the investment, I can get a 21" frame and move everything over. What I want to know: Is this a practical idea? I don't know how interchangeable parts are and I need to find the balance between having something that can get me into the sport without sinking a whole paycheck before I know it's something I'll stick with.