...and I don't play WotR, so...
If you get the Mines of Moria set you'll have your goblins, a troll and all the rules, but only a few selected profiles. All the profiles you'll need for goblins, trolls and dwarves are in the Khazad-dum sourcebook, though some new goblin and dwarf models have recently been released, and the stats are available in the previous issue of White Dwarf.
All the profiles you'll need for Gondor are in the Gondor in Flames sourcebook, while Minas Morgul requires the Mordor sourcebook. Elves are a different matter, depends what kind you want.
For Gondor models, you can't go wrong getting a box of WoMT, KoMT, a command blister, and Faramir. I suggest him because he's a solid mid-range hero, capable but not too expensive. For Mordor there are a few more choices, but for just starting out, a box of Orcs and a box of warg riders is good for warriors. To lead your riders I'd suggest Sharku...though he's technically part of Isengard's list, he can be allied in. A command blister is handy, along with named mid-range heroes like Gorbag. Evil also benefits from having a Shaman, and a ringwraith (even a generic one) can be pretty handy.
That's enough to start with to learn the game, at least with SBG. With WotR you might need a couple more warrior boxes on each side.
You might want to think a bit about what kind of game style you prefer. WotR uses trays and allows you to field huge armies (which of course cost a lot and take a lot of time to build), though terrain becomes more limiting. It's a very different style of play from SBG which allows you to move single models around much more varied terrain, but tends to bog down at around 1000 points. IMHO, WotR is too abstract, and SBG is more personal and story-driven, but that's a matter of taste.
If you pick one over the other you can save money...you either won't need the WotR book, or you won't need the SBG sourcebooks. Hope that helps...