I completely agree that there's nothing like shooting film but I have to disagree with some of the other things you say.
Film shooting or developing is not imperfect or surprising if you know what you are doing. On the other hand, when something does happen one doesn't expect, with film it's usually very interesting.
I am not sure what chemicals you feel have gotten expensive. I use powdered chems and concentrated liquid developer and I don't think the prices have changed much in the 10 years I've been shooting. If anything, it's just been inflation. I also haven't seen any of the Kodak chemicals that I've ever used become discontinued. I know many many people who still happily and easily process their own film so I just don't understand why you think it's not worth it. I purchase my supplies between BHPhoto.com, Adorama.com and FreestylePhoto.biz and get fantastic prices on them, even with shipping costs.
Instant cameras are fun but they don't produce anywhere near the image quality of a Pentax K1000 and you can actually make more imperfect "accidents" with a K1000 and some creativity than you can with non-peel apart instant film. Peel apart instant film can offer a lot of creativity too though!
The Polaroid 300 is Absolutely nothing more than a Fuji Instax Mini 7 that has been repainted with the Polaroid logo. It even takes the same film. So if you are considering this camera, go with the Fuji. I bought my Instax Mini camera and the film from Adorama.com. Seems to be the cheapest.
Diana's are cool cameras because you can get an instant back for them as well as other Lomo accessories that transform them for many different effects.
Simply using cheap Bently and Meikai plastic $5 cameras can give you some of the same looks as Lomo cameras too but not on instant film.
Have fun and don't forget about your Pentax and processing yourself!