I don't know if this was made very clear yet, though it was mentioned, but on any Pentax DSLR except for the new K-1, all your lenses will have a "crop factor", meaning, only the center of the image will be used, and your lenses become "longer" because the sensor is smaller than the size of the film you are used to. On the K-1, the sensor size is the same size as film, so you would see no difference, but it costs quite a bit - and that is the same with all other cameras with a big sensor size, unless they are very old like the original Canon 5D - and a well kept 5D with a low shutter count would still cost you around 400 dollars at least (and you would need Canon lenses, too...).
So: in other words, your 50mm lens becomes a 75mm lens on all Pentax DSLRs except the K-1. The 28mm lens becomes a 42mm. If you have a 135mm lens, it would now be a 200mm lens. Would you be willing to live with that? You would probably have to add a new wide lens if you are used to shooting with a 24 or 28mm lens, for example. And in this case, zooms are more affordable than primes... I bought a Pentax 16-45mm f4 lens, which by all accounts is a pretty good lens and covers all my wide angle needs, for 120 dollars. I also have a Tokina 19-35mm which is sort of my backup, which I bought for 90 dollars.
Regarding cameras: if you are shooting the K1000 right now, you probably don't shoot anything over ISO 800, but correct me if I am wrong. In that case, my recommendation would be the Pentax K10D. If you expose correctly, you can get great looking pictures up to ISO 800, and ISO 400 and lower look fantastic. It has a lower megapixel count (10MP) than newer cameras, but still capable of printing up to A3 size. The K100D which was also recommended, is very similar and the main difference is that the K100D only has 6MP. Both are capable of great pictures but considering the difference in cost is small, I would get the K10D.
KEH.com currently has a couple K10D's in the 109-115 dollar range, both in EX and EX+ condition. In other words, it's cheaper than a similar condition K1000! They don't have K100D's at KEH. BHPhoto.com has a K100D for 100 dollars and it's in their 8+ condition which would probably be like KEH's "BGN" condition (BGN=bargain, meaning well used).
I got a K10D recently (and paid a bit more because it only had 4000 shutter actuations) and it's now my main camera for all my fun pictures (for family pictures we mostly use my wife's K-r). Here's what I got with it in the month or so that I've had it:
https://www.flickr.com/search/?user_id=11375085%40N02&sort=date-taken-desc&t...10d&view_all=1
Note that all pictures I took with the K10D were taken with old film lenses. I have a couple of affordable and good DA (digital) era Pentax lenses but they stay with my wife's K-r.
For about 100 bucks, I would get the K10D, and also try the mail-in film development services. You might end up enjoying both systems
Edit: I forgot to mention, the K10D and K100D are similar because they use a different sensor technology than more recent DSLRs. It's called a CCD sensor, and the K10D was one of the last Pentax cameras that had it (K200D and K2000/K-m also have it but they have smaller viewfinders that I would not recommend for manual focusing). The older CCD sensor has a more film-like behavior that is well documented in a forum thread called the K10D Club:
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/242738-k10d-club.html
Newer CMOS sensors are great too, and they can do video much better which is one of the main reasons why the technology changed from CCD to CMOS, but a lot of people who are used to shooting film like the CCD look.