I'm going to shoot a friend's wedding in two weeks. I'm an available light shooter at heart, but now I need a flash. Never entered this part of photography before, so I'm completely blind. I think I need a flash that act like the build-in flash, an automatic metering flash. I'm going to use the flash scarcely, so I'm thinking about sub-$100 AF flash unit, which will more likely to be a non-Pentax brand.
I've been looking for a Pentax dedicated flash for my K20D, and found few things are just beyond my understanding: the TTL or P-TTL thingy. I know that my camera works only on P-TTL flash, not TTL, but I don't know what it means exactly

I learned that the TTL is through the lens metering, so what about P-TTL?
I see that there are plenty Pentax dedicated flash available on the market. I'm interested on items like Nissin 34Di/34iaf, Rokinon D870AF, Vivitar 730 AF, but I see that they are listed as TTL flash. Does this means that they won't work in auto with my K20D?