Lots of people LOVE the cheapie cling mount stamps. I have some and they are good, for me I don't buy a stamp unless I see a set or one I don't have and NEED to get it. I probly have 4000 stamps, never counted, but $ is tight so even $1 can be an issue sometimes
I have never used "Ink It Up" so can't help there.
i have copper ink it up and i like it just fine. i bought it at a clearance store and it gives pretty results with a warm shimmer. i don't have experience with any other IIU colors.
look on youtube for "everyday scrapper" - she has a good video on conditioning clear stamps. when i come home with new stamps, first thing i do is scuff them lightly with a plain white (or pink) eraser. i got mine from michael's but any eraser would work. you can also use very fine sanding block but i love the way an eraser works, and i know it's gentle. when you're done, you can tell because it leaves just that - a bit of a scuffed finish on the raised surfaces of the stamps. then take a soft toothbrush and some gentle liquid soap and gently wash the stamp(s), and rinse them in hot water and let them air dry - they'll be all ready to use.
if you ink up a stamp you've conditioned and you're not satisfied with the way it's holding the ink, hit it again with the eraser, or go lightly across it with a very fine sandpaper/sanding block/nail file - very gently - then wash as above.