The dynamics of combat are totally different. D20 is a grind, hit point attrition, a PC can manage risk. RM can turn against you with one bad roll (and especially with two). Every fight could kill you. Especially while the players are adjusting it would not hurt to give them a limited number of rerolls / fate points. (The version I like best is if the player expends one (e.g. to reroll a fatal critical against their character), the NPC receives it for their own use....)
RMSS has a more complicated character generation system than RMU. I think my spreadsheet for making characters is in the Vault on this site, or you can download it from my site:
http://www.madreporite.com/rpg/rolemast.zip Once you know the system it's not difficult to make a character (whereas with D20, I felt like I always needed to do an extensive research project cross-referencing feats and abilities across scores of books) but especially at first it feels like there are a lot of moving parts. If you're going to run a game for people who haven't played RM before, make some characters yourself so you know the system and can guide them through it. You can use them as important NPCs so the effort isn't wasted.
I would also suggest printing out or photocopying the relevant pages for each character. Their character sheet, but also each of their spell lists, maybe even the pages describing how their key skills work.
I have not personally used ERA but it's pretty nice to have software tracking combat and injuries. If you don't, also print/copy the attack tables for their weapons and critical charts. Some people like to assign a player to do those look-ups during combat to keep things moving faster for the GM. It probably makes sense to try this first by hand before using software, you'll get a better understanding of how it's actually working.
There's tons of other advice scattered throughout the forums. Feel free to bring any questions back here as they come up.
