Here's my highly subjective opinion.
1. I have UV on my main walkabout lens. I don't have it on any of the other two lenses. I'd keep it on the lens that's on camera most of time - to reduce dust and fingerprints and cleaning. However if you don't have it that's probably OK as well.
2. CPL helps to get richer sky colors on a bright day, or to cut the glare & reflection in say water. I see more overall use for a CPL than for ND or GND filter. However if you don' intend to keep your lens either hold off buying a CPL until you have the right lens, or buy the big CPL and an adaptor. You can use 67mm or bigger CPL on a 58mm Canon lens but not the other way around.
3. HVstar.com is a great place to buy filters cheaply.
Good luck !
