My suggestion would just to buy the base cards if you want to play with 5-6 players. If you buy another base set, you'll be stuck with extra cards.
If you're looking for a good expansion for newbies, my recommendations are Intrigue and Prosperity.
You can buy a set of base cards individually!
Rio Grande Games Dominion Base Cards https://www.amazon.com/dp/B007MKLX84/ref=cm_sw_r_cp_api_glt_i_5339P9W1P8667HB1Y37A
if you never bought the "base cards" release from last year, this might be a good occasion for that. it's got new art for the treasures, curses, and victory cards. they're nice. $11.58 on amazon (and eligible for prime): http://smile.amazon.com/Rio-Grande-Games-RGG-471/dp/B007MKLX84/
(or 10.99 on CSI but you gotta worry about shipping)
If you don't, the new style base cards are pretty cheap.