Hi Marie13, the two ladies I dive with often, both use steel HP80. The rest of the boys are on at least steel HP100 and I'm on LP&HP 120's we all end the dive at about the same time with about the same amount of gas. I didn't want to be the one thumbing the dive all the time so I got bigger tanks.
B) the DIN tank valves should have a yoke threaded insert that comes with them, the old Worthington tanks came with them. So you need nothing to dive your new tanks.
C) get a tank that can be filled by your local shop, not all shops can fill HP tanks fully, some shops can only fill to 3000PSI.
D) see if you can try some of these tanks before you buy, then once you commit to a tank size you may want to buy 2 or 3 that have following serial numbers, so they are exactly the same height, and shape. It makes them more valuable to someone who wants to double them up.