Direct Plastics is the cheapest I've come across for HDPE. Much cheaper than buying off eBay. 20mm HDPE all over could make you a bit too heavy as dotDominic said, but you can always take some big weight-saving chunks out of your bulkheads and still have adequate support.

Honestly I have no idea how many fights you're going to get out of one refill, depends how much pressure you're using and how aggressive you are. I think most if not all events have a place where you can fill up your CO2 between fights so it shouldn't really matter as long as you can last one fight.

Unfortunately I bought my LiPo charger second-hand and it's discontinued now. I don't really have the experience with anything that's currently on the market.