RM2 had way more supplements than RMSS/FRP and RMC is RM2. Getting pdfs of the RM2 supplements will have to wait in some cases, though, but it doesn't take too long to get hard copies of most of the RM2 stuff.
Exactly. RM2 had
way more stuff put out for it, which eventually became the problem. There was little to no oversight on the organization and balance of all the supplements and, unless you were pretty well versed in the system and had a good handle on how to make sure things didn't get out of hand, it started to cause problems. RMC is RM2 that has been 'fixed', but only a portion of all the stuff published has gone through the filter... so using RM2 stuff as-is with it ends up causing the same problem for someone new to RM in general.
You have the same carry-over from RMSS to RMFRP, but there was more control by the designers when it came to organization and balance in RMSS. RMSS put out a decent amount of material before they decided to revamp to RMFRP, although not as much as RM2. While I believe a system should have a new version put out at least every 7-10 years I honestly didn't see the point of RMFRP. Existing RM users didn't really need it, luring new customers needed a system which would be perceived as simpler (which I do not think RMFRP accomplished), and there was still a decent amount of stuff that could have been published under RMSS. I suspect ICE just didn't have the resources to come up with new material, so they opted for a revamp instead.
Anyhow, so in my opinion, the best place for someone to start fresh with RM is to use RMSS or RMFRP and, once familiar with the system, start converting over RM2/RMC material. You have better balance and organization, you have a decent amount of material, then once you really know your stuff start stealing material from the other versions of the system that you like. We converted a lot of RM2 stuff into a highly customized D&D for years. Might work out well as some more of the RM2 stuff might get put through the editing filter in the mean time also.
Now, if someone was already an RM2 user and knows the system, that's a whole different story. Go with RMC at that point and convert any cool stuff from RMSS/RMFRP instead.