PUL is a fabric that's waterproof but more breathable than PVC. PUL wraps are heaps easier to use than the pull-on fluffies or PVC pants IMO, but they are more expensive - about $15 a pop. Worth it!
Yup, you buy them online mountain mummy. Baby beehinds (if those are what you're considering) have a website and they do have some packages where you save a little bit of $$. I would think if you are washing a lot then 25-30 would be enough. (Depends on how much they wee and poo!) Personally, I'd go for the bamboo rather than hemp. Heaps more absorbent and softer, but take a little longer to dry.
Not sure if it will help with the TT... Particularly if he already knows the feeling of being wet and knows what it's about, KWIM?
But personally, I'm already thrilled with the money I've saved using them, even just part time. I find that it is more work, but worthwhile.

ETA: Whoa, sorry I obviously took ages to write that response! Sorry for the repeat info