I would get the core book or look at one in a shop near you to see if you like it. Then I would read the descriptions on the other products in the online store to see if you would like to have them in your game.
I would also do a little ground work ahead of time since I generally have had trouble adding in things as they come out to my game world.
IMO since I like RMSS/FRP is buy the RMSS Core, Spell Law and Arms Law as PDF's. Then you can print out the pages you need for each charcter. Also IMO RMSS, SL and AL are easyier on the pocket book then RMFRP, SL:C, SL:E, SL:M, AL and monster book. Oh I forgot to mention the creature book in the begining it will help a lot but as you game you get used to the rules it is very easy to make up your own.
Have Fun
MDC