If you are looking at older models (no video or very basic video), it will pay to look for what were the higher models at the time, because they had more rugged construction, will be more durable and have a longer shutter life. For example, in the Pentax range, in 2006 the K10D was a higher model and the K100D was entry-level and half the price, but in the secondhand market now there will be very little between them.
Here is a tool for comparing different Pentax models:
Pentax K10D vs. Pentax K20D vs. Pentax K-7 - Pentax Camera Comparison - PentaxForums.com
I have selected the K10D, K20D and K-7, as these were the top ("flagship") Pentax models of their time. You can see that the K10D had a 10 megapixel CCD sensor, whereas the K20D and K-7 had a 14 megapixel CMOS sensor (If you need to crop a photo - ie cut bits off it - the extra megapixels are an advantage) and added live view (that is, you could use the rear screen to focus). The K-7 introduced (basic) video capacity, as well as having a better processor, better metering and better autofocus and a number of other desirable features. But if you are just going on image quality, results are going to be virtually indistinguishable from the K20D.
Many people loved the K10D. Some people argue that the CCD sensor produced warmer colours. Look at this thread to see what it could do:
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/242738-k10d-club.html
On a quick look on Gumtree UK there are plenty of K10D cameras for less than 100 pounds. People seem to be asking 200 pounds or more for a K20D or K-7 (which seems unrealistic to me).
If that is representative of prices, maybe
look for a K10D, and save the difference for lenses. It does make sense to reserve half your budget for lenses and accessories.
At less than 100 pounds the K10D is a good buy. Here's one for 80 pounds with a lens:
https://www.gumtree.com/p/digital-cameras/pentax-k10d-slr-digital-camera/1199192172 And here is another for 100 pounds with an 18-55 lens, two batteries and a case:
https://www.gumtree.com/p/digital-cameras/pentax-k10-d-camera-equipment-/1194317065
I don't think you could go wrong with these. The sellers might even come down a bit and offer free postage. I wouldn't hesitate to buy an item like this on Gumtree; you'll pay a lot more in a shop or even on ebay (because the seller needs to recover their fees).
Don't forget you will need one or more SD cards, but they wouldn't need to be particularly fast or large capacity. (In fact from memory my K100D Super couldn't use large-capacity cards - check the manual before buying.)
Note that all of these cameras had in-body image stabilisation, unlike almost all of their contermporary rivals. At the risk of sounding like a broken record (sorry, that's a metaphor that won't mean much to a 14yo!), that is an invaluable advantage, particularly shooting handheld.