I'm more on the RMSS side. In terms of books it depends on what you want. With
RMFRP core book you have everything you need for a couple of professions up to lvl. 10, just like you were used to with MERP. If you want more professions you'd need
Character Law, if you want all the spells to higher levels and for the new professions you'd need indeed three Spell Law books (
of Channeling,
of Essence and
of Mentalism). And if you want more details for the weapons then you'd need
Arms Law.
So it's indeed more books you'd require if you go the RMSS/RMFRP path with all professions and all spells. So if budget is your constraint, then go for RMC. I personally think that RMSS, with the similiar skills rules, Power Point Development (PP Dev) and Body Development progressions per race, PP Dev as separate skill etc., is an improvement over the older RM2/RMC rules. But the rules for similiar skills, implemented via rank development for a skill category (e.g. Athletic - Gymnastics plus development for the individual skills (e.g. Climbing) in conjunction with the increased list of skills leads to more time that is required for character creation. Again, if you don't want the level of detail, then go for RMC. Otherwise I'd suggest to go for RMSS/RMFRP.
Just my 2 cents