Buy what you like.
Trust me, it can't be stressed enough. If you buy everything, even if you don't like it, you'll go nuts and go broke.
LEs are usually a safe bet for buying multiples to trade. They may be a little more expensive than open edition or rack pins, but you can use them to trade for a LE pin you might have missed.
Supply and demand plays a huge role. Not all LEs of the same edition size are equal, though. Wet Paint and Black Heritage Month are both LE 1000, but the demand for Wet Paint is WAY higher than Black Heritage Month. Likewise, there are retired rack pins like Stinky Pete that are in high demand.
If you're looking for pins to trade with Cast Members -- the pins from the Disney Stores are a bit cheaper plus you can get a Disney Club discount. If you drive down I-95 to WDW, there's a Disney Character Warehouse (I think that's the name) that have a bunch of the Countdown to the Millennium pins for around $4.00. Check eBay for lots that have low per-pin average prices, and Cheappins.com also has pretty good deals.
And don't forget ... buy what you like.
And if you buy something and later decide you don't like it, then it's a trader.