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If it's primes you're looking for, I'd say best to stick with Pentax. In manual focus, look for Pentax 'A' series (the front of the lens will read SMC Pentax-A)... in 28mm, 35mm, or 50mm. Anything much longer could be too long for those inside candid shots. Plus lenses within those focal lengths should be the most reasonably priced.
The cheapest way to get a fast lens is usually with a 50mm... cheapest would be a 50mm f/2, followed by a 50mm f/1.7, 50mm f/1.4 and finally the most expensive 50mm f/1.2. The f/1.7 or f/1.4 are probably best in terms of performance for money.
By the way, before buying anything, I recommend taking your kit lens (I assume you have the 18-55mm?), and try using it at various different focal lengths (28mm, 35mm, 50mm)... try walking around shooting the kind of shots you want the new lens for; this may help you decide what would be the best focal length.