I just started cycling about 3 months ago and I would like to learn how to do my own maintenance. My roommate brought home a 5 speed MTB that isnt in great shape so I figured it would be good for me to practice on. The frame is in good shape but the drive train is a little rusted out. I wanted to just grab some cheap parts off Amazon or Ebay and learn to install them. What type of measurements do I need to take to determine what type of parts I can place on the bike? Such as finding a crank that fits

It currently has Shimano stuff with no type of model number or anything on them. The crank has no ID on it at all. I was looking at Alivio stuff on Amazon and I can get a crank, FD, RD, and shifters for less than 150 bucks total. Yeah, I will need some other parts as well but this will get me started. Also, I'm sure I could find even cheaper parts, such as this Shimano Tourney line. I'm not worried about being able to take the bike on long ride in the future or anything I just want to learn how to service a bike but I would like to work with parts that arent rusted out to bad.

Any suggestions? Thanks in advance for any replies.