I agree with all of the above. As a rule, I'd keep a light crystal (20-40L, or carapils) on hand. Balanced with a dark crystal (80-120L) - I've made good use of a 10 pound purchase of Crisp 77 for a number of beers, nice stuff and not the typical American 60L flavor. I like to use a bit of melanoidan or aromatic for extra maltiness - but the Munich recommended by all will get you there, too. When it comes to dark malts, I love the pale chocolate.
Good luck on keeping it simple, though. Soon, you'll probably have a couple dozen bags of miscellaneous ingredients. Just keep 'em labeled!