There are a ton of good used lenses out there. IMHO, it's what makes Pentax such a bargain as a camera system.
One lens I'll recommend is a 135mm prime. I have 2 right now (need to sell one). One is a Takumar (made by Asahi-Pentax) 135mm/f2.5 fully manual lens. The other is a Pentax SMC-A 135mm/f2.8 ("A" designation means auto aperture). You can find similar lenses on ebay for well under $100, usually around $60-75. I actually bought a Sears multicoated 135mm/f2.8 lens at a camera show for $5 and it was a nice performer. I sold it to a student for the same $5.
135mm on your camera equates to about 200mm, which is a reasonable telephoto. The f2.8 means it's pretty fast, so you can take low light shots. For the money, it's a fun, useful lens. I think it would make a great concert lens unless you're shooting from the rafters.
I tend to just buy things without a real plan. The smarter path would probably be to avoid buying some much and focus on getting one nicer lens, like a 300mm/f4 telephoto. But I've had fun with the lenses I've bought, so I guess it's okay.
Here's the example I always post of a shot with my Pentax 135/f2.8 taken at an NBA game. ISO200, f4.0, 1/160sec. I was at the other end of the floor about 20 rows up, so I got pretty good reach.