Amie,
With your budget and intentions, there are two distinct paths IMHO:
(a) prime lens(es) or
(b) zoom lens(es).
You need to think carefully of your needs. Do you want convenience? Do you aim for IQ? What focal length range do you need? Do you want to shoot in low light conditions?
The zoom lens choice is for convenience and Adam pointed you to the obvious: 55-300mm to complement your 18-50mm lens, or upgrading your kit lens to a 17-70mm f2.8 zoom lens. Both options are worthwhile to consider depending upon your needs.
The first path (prime lens) will bring you a major enhancement in image quality (IQ). Pentax offer a broad range of new and older (compatible) prime lenses which are outstanding. Often the first prime would be a fast 50: i.e., 50mm f1.4. Such a focal length (50mm) is great for portrait while the large aperture (or low f, f1.4) allows to shoot in low light conditions without flash. Great for indoor portrait for example, of for shooting outdoor at dusk and dawn. The FA50mm f1.4 is a good choice for a AF lens and you may find cheaper MF lenses on the market place. Generally most prime lenses will give you better IQ than a zoom lens for the same price.
Zoom versus prime? The choice is challenging. I have 2 zoom lenses and 5 primes. My first lens was a zoom and my second lens was a fast 50 (VL 58mm f1.4). Everyone if different and I hope that the comment may help.