There are three primary versions of RM to look at...
RM1 and RM2
This is the original Rolemaster. It was started as an "add on" product (essentially RM1) to other systems and eventually grew into a formal system (essentially RM2). It's hard to get your hands on everything you'd need to make a go of this version, so it's pretty much of the old-timer RM fans. It's also a little difficult to manage due to a seeming lack of organization if you're new to RM.
RMSS (Rolemaster Standard System) and RMFRP (Rolemaster Fantasy Role Playing)
This is the first major revamp of the system. I, and many others, think it's better presented, organized and balanced if you're just coming into Rolemaster fresh. It's not as hard to find as the RM1/RM2 stuff, but it's hard to find everything at a reasonable price (some of it you'll have to track down in stores - probably online ones - or get off ebay). The primary difference between RMSS and RMFRP is the main rule book, find one of them and the rest should easily fit in with either I think.
This is what I use. You'll want the...
1. Main Rule Book
2. Spell Law(s) - either one main one with all spell types or three divided into Channeling, Essence
and Mentalism called "Spell Law... of Channeling" and so on.
3. Arms Law
4. Creatures and Monsters
Then look for the companions: Essence, Mentalism, Channeling. Then Treasure Law, Character Law or Talent Law, and maybe Gamemaster Law.
RMC (Rolemaster Classic)
This is basically a cleaned up and somewhat altered version of RM2 republished for the RM old timers. I don't know if it'll be any easier or harder to find than RMSS/RMFRP.
Then there's RMU. If you want a fully functioning system this might not be for you until it's actually done (there's no creatures and monsters beta book yet for example). However, feedback from a new Rolemaster user to those developing it would probably be incredibly useful to them as one of the future goals is to draw in new players.