Hello and welcome to the forums - glad to have you with us
I think you're doing the right thing by starting off with an inexpensive, older DSLR. The newer models have more bells and whistles, better image quality and much-improved high ISO performance (which would be useful for your night photography) - however, you can learn the ropes with the K100D and, if you enjoy it, look to upgrade later.
You're quite correct that older Pentax K-mount lenses (and even earlier M42-mount lenses with an appropriate adapter) can be used on the K100D and all other Pentax DSLRs.
We have an active Marketplace here on the forums where you can often find some very good deals on excellent lenses (and cameras, if and when you decide to step up a notch or two). Many of the members selling on the Marketplace have a lot of background on these forums, and you'll usually find that descriptions of equipment for sale are a lot more accurate and honest than those on auction sites etc.
Ask all the questions you like. There's almost always someone online who can help - the forums are very active