Hey, welcome!
I'll try to write down the key concepts for old lenses on your Pentax DSLR
a) Zoom vs. prime: Lenses with a fixed focal length are called prime lenses. They have one one field of view and it cannot be changed. They (should) give higher quality rendering and better low light performance. Zoom lenses are the ones with a "zoom ring" where you can change the effective focal length. So you can zoom out or zoom in. Do not get this confused with "tele lenses" which just make faraway objects appear close (tele lenses are the opposite of wide angle lenses). Modern day zoom lenses are very complex and can deliver pretty good image quality! But if you are buying lenses older than 15 years, you should probably avoid zoom lenses and stick to primes. This gives you a better chance of getting good image quality
b) Manual vs. automatic: At one point, autofocus was invented. Lenses made before that, do not have the AF mechanism, so they need to be focused manually, by hand. Even aperture control was invented at one point, so lenses before the Pentax A series do not allow you to control the lens aperture on the camera - you need to control it by hand on the lens itself. The Pentax K and M series (and older) need to have aperture adjusted on the lens, which means continuous metering is also not possible. Basically, if you buy a lens with the letter A on the aperture ring, it will be easier to use. Pentax A series lenses have automatic aperture control, but no autofocus. These are often good deals and allow you some more automation than M series lenses.
c) Wide angle: In the past, lenses wider than around 24mm were rather rare and not as good as modern day wide angle lenses. So you should probably avoid those.
d) 35mm, 50mm! These focal lengths were very common and popular in the past, so they are not too expensive and deliver good image quality. Especially 50mm lenses. I have a Pentax M 50mm f1.7 that I bought for under $60 and it is a great lens. I think the optical design is almost the same as the modern Pentax DA 50mm f1.8 - but the DA has modern lens coatings and automation like AF.
e) lens hood, filters: The older the lens is, the more primitive its lens coatings are. Especially on digital cameras, this can affect the image quality. To help with this, buy a lens hood for the lens. It can make a big difference, especially in sunlight. The lens hood must be appropriate for the lens' focal length. You can find many cheap chinese knock offs on the ebay or other such websites, often for under $20 and with free shipping. Regarding filters, you probably don't need anything. UV filters were important on film, but are practically useless on digital cameras (some use them for "protection" but some don't even accept that). Polarizing filters might be useful for landscape photography and flower photography, but no need to rush. There are also many threads about filters if you want to read up
A good resource is the Pentax forums lens review database, where you can check how others feel about a certain lens:
Pentax Lenses - Pentax Lens Reviews & Lens Database
Maybe you already know a lot of this, but this is the basic info that will get you started in the exciting world of legacy lenses.
tl:dr; To start off, I would suggest you buy older 50mm or 55mm lens (preferably with f-number smaller than f2) for portraits and a 24mm lens for landscapes. The ones with the letter "A" on the aperture ring, will be easier to use, but will cost a little more than the ones with manual aperture rings.
But! I hope you get the kit DA 18-55mm lens with your camera. Its not an amazing lens, but it is a good learning tool. And often its very handy to have a nice zoom range and decent AF. Especially if you get the WR version of the lens, then you can go out in poor weather! Remember that the lens has to be WR, too if you want to go out in rain
Also, here is a great thread to help you with the technical part of actually using old manual lenses, once you actually have them:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/110657-how-use-me...k-x-k-7-a.html