Older Wacom digitizers (especially the Intuos 3 series) seem to be available pretty cheaply used (I got an A4 version for 25€ and a A5 version for 20€ some time ago).
As for alternatives there are quite a few but most of which I tried didn't come close to Wacom (I heard some good things about XP-Pen and Huion but I never tried those two).
The digitizers yes, but the pens (ZP series) are rather expensive, more than the KP-series for the Intuos 4 tablets. If you can find an Intuos 3 with a pen at a reasonable price, it's a bargain. They are mostly 4:3 though.