I'm a huge fan of subpanels, and in particular, I'm a huge fan of BIG subpanels. Now I see where you totaled up your loads and got 65A, and you're like "70A is a panel size". Now I'm not sure where you think you have to go exactly 70A, or just are cheap. First, you're allowed to go bigger, and second, there are lots of great places to be cheap in electrical (in fact I'm about to introduce you to one, but panels are not the place.
You want a nice panel, and what defines "nice" is lots and lots of spaces. Your math may say you need a 6-space (or even a 4-space using double-stuff breakers) - I'm telling you to get a 24-*space*.
Go insane. Spaces are dirt cheap (when speccing a panel) - but boy, when you're trying to add something later, and just don't have the spaces, regrets are expensive. Set it up so you never have that problem. Ever.
You don't need a main breaker in a subpanel - they're often used as the cheap way to get mandatory disconnect switches, but you don't need that here. So a main-lug panel will be fine.
The bus current rating (e.g. 70A) is like the speed rating on tires - you'd rather be driving 112 mph rated tires than 85 mph rated... right? More margin of safety. So don't worry about busing that is much larger than 70A. Can't hurt, gives you safety margin. Like I say I'd use a 24-space 200A bussed panel myself. The advantage is huge, and the coin isn't enough to care about.
As far as current factoring, since cooktop and oven are involved, the math gets really weird. I'm not an expert in that but someone will speak up. Giving them a full 45 amps of capacity might be overkill, but it's a solid choice. On the water heater, whatever the instructions say, you have to do that. Otherwise read the amps off the nameplate, multiply by 125% (i.e. add 25% more) and round up to the next available breaker. I usually expect 30A for a water heater, so I would run #10 wire in case the *next* water heater is 30A.
Now, on the feeder... you say none of your loads need neutral. It'd technically be possible to run this subpanel with no neutral whatsoever. Whenever you wire a 240V-only panel with no neutral, it's inevitable that someone will come along, go "ooh, subpanel!" and slap a 120V circuit in there. They will just assume it's a pre-1989 subpanel with common neutral-ground, and slap the neutral on the ground bar. This is inevitable. That creates a dangerous situation (which is why it was outlawed), so I strongly advise bringing neutral to this panel even though you don't need it right now. If you don't want to do that, I understand, but then I really do advise making it a 4-6-space panel and totally filling it, using a separate ground bar (so the neutral bar is EMPTY), and stick warning labels inside and out. Since it is 240V-only you do not need common-trip breakers.
Ah, but look at me, spending your money. I promised savings!
Aluminum wire. Seriously. It's true that on small branch circuits, it played badly with copper terminations. It has always worked well for inter-panel feeder. (not least, because the lugs are aluminum lol. Aluminum lugs play well with both Cu and Al wire). I laugh when people insist on spending 3x as much on copper feeder "to avoid dissimilar metal problems" then land it on aluminum lugs and don't even realize it.
If it were me, I'd run #1 Al wire so I could run full 100A. Otherwise for 65A actual load, you can run #4 Al or #6 Cu wire (other than NM-B or UF-B; you need wire that is allowed 75C thermal rating). This can be breakered at 70A in the main panel. Price it all 3 ways (#1Al #4Al #6Cu but remember the #6 cannot be NM-B).
