I entertain regularly, and was a bartender for a number of years... hope I'm not forgetting something important in this post, but I probably am...
having "a fully stocked bar" doesn't have to break you. You might be able to get away with having one for as little as $300 if you spend wisely... or maybe even as low as $200 if you do it on the cheap.
Aside from 'whiskey'...
Vodka (don't have to get too pricey here unless you like to display bottles - it's usually used as a mixer - I always have a handle of Ketel One or Skyy on hand, myself - cheaper and not many detectable fusels). Tequila (Cabo Wabo or Patron, imho, unless you wanna spend $80+ on a bottle), Rum (I always stock both light/golden and dark so that Mai Tai's are an option. Go Mount Gay for the light (best rum for its price by far, I think)... Myer's for the dark), _maybe_ Gin (not a lot of peeps drink it, but having at least a bottle of Tanquerey on hand is good... if you know someone that drinks gin and wanna spend a bit more, either find out what they like (gin flavors can differ greatly) or stock a solid mid-shelf gin - like Bombay Sapphire). You'lll also want dry vermouth (it's cheap) for martinis. That does it for the _must haves_, imho.
Also a good idea to keep some cordials on hand... my top 3 recommendations: Bailey's Irish Cream, Kahlua and Amaretto... as those that prefer sweet drinks can drink them straight up (if not with milk-mixers, like a White Russian or Colorado Bulldog), or you can use either of them to flavor coffee for an after-dinner drink.
You'll also want, for mixing, at least coke, sprite, tonic (I always keep a few mixer bottles of soda (can pick up at some liquor stores like BevMo) in the bar), a few red bulls, OJ, Pineapple Juice, Cranberry Juice (juices frozen or canned, if you don't drink them regularly), Margarita Mix (or make your own sour mix if you wanna do 'em from scratch), Milk, and Sugar. Pick up some Rose's Grenadine and Lime (these are cheap - like $4 - $8 a bottle), as well.
if you wanna go the extra step (not necessary, though, unless you're throwing a party, maybe) , always have salt, limes, oranges, lemons, marishino cherries, celery (if you don't keep this regularly, you only need it for bloody mary's)... maybe a pineapple and some mint... on hand for garnishes in addition to what's listed above.
Add a selection of wine and beer, and you, at this point, I think you would have a basic "full bar."
Of course, there's always room to flesh out your bar significantly... you can adopt more varieties of whiskey (scotch drinkers would probably appreciate first dibbs here, but a bottle of Bushmill's Black would probably help our the cause signifiantly, too), more top-shelf alcohols, more cordials (Sambuca and Galliano are two of my personal 'not as common' faves), digestives (Jagermeister and Absinthe are party favorites), sweet vermouth for perfect or sweet manhattans... and some of those come with their own set of things you need to stock (for instance, Sambuca is _always_ served in a snifter with 3 roasted coffee beans dropped into it)... and you can always have countless varieties of just about any specific spirit, but, you wanted basic, and, provided you're following all but the last pararagraph in this post, I think you're more than covered.