We need a budget to work with please.
That said, I'd get a Nikon D40 or a Nikon D70 (around $300 used), add a 35mm f/1.8 DX lens ($200 new, if you can find one) for short-distance stuff like snapshots at a party or on a vacation, which brings you to $500.
That's not going to cut it for shooting from the back of the auditorium. For that, you need a long (because you're far from the stage), fast (because indoor light is pretty dim, photographically speaking) lens, which is quite expensive. Ideally, you'd look at the 70-200 f/2.8, which is $1700 or so, or a 300mm f/4 (for which you'll need a tripod or monopod), which is $1400 new, shave off a few hundred for used. And of course, once you get longer than that the prices rise exponentially (over 8 grand for a fast 600mm lens)
If you choose to get the D70, then you can get a AF 85mm f/1.8D (which I see for $350 new on
eBay), but even that's probably going to be a little short. Or, you can get the 180mm f/2.8D (about $350-$400 off
eBay, used)... anything longer and you're looking at some big money to put up with, to stick with fast lenses.
If you choose to get the D40 (and the following lenses will work on the D70 as well), you can try either the 55-200 VR lens ($150 or so used) or the 70-300 VR (I see one for $440 on eBay used). However, both of these lenses are pretty slow, and you can't just take a longer exposure with a tripod because then whatever's happening on stage will be blurred. I include these for completeness but they're not recommended, unless you have a very low budget. Either way, see if you can find a local camera store, and rent a 70-300 G ($100 used, but this doesn't include that VR so you will *need* a $300 tripod) and go to the auditorium in question when it's empty. Sit where you normally sit, aim at the stage, and try to see through the blur (since it's not on a tripod) which focal length you can live with, then return it and buy the fast lens you need.
Overall, expect to pay at least a grand for a basic setup.
edit: about Pentax - if you're going to buy 1 or 2 lenses once and never again, and can find what you need from Pentax, then sure go with them. However, for any camera system other than Nikon or Canon, you will find that there aren't too many after-market lenses and not too many ways to expand after your initial purchase. With Nikon or Canon, you can go into flea markets or camera stores and pick up a huge amount of used equipment (as demonstrated above) for a pretty good price... not something you can do when nobody owns Pentax equipment.