Steel is usually the choice particularly if you have any suit on. If you are that close to negative AL or cutout steel might be better. It also gives you more flexibility in where you allocate ballast in terms of trim. Just please do work to trim yourself out by putting lead where you need it. Life diving is much easier if you float horizontal when not actively doing anything. More drag vertically so depth control is easier, less drag and oriented to kick in the direction you likely want to move, horizontally.
On BP/W basics, you might read the first parts of this:
Beginners Guide To BP/W
One of our bigger guys,
@tbone1004, might advise you more on plate sizes for being 6’1” 180.
Tanks play in as well. AL80s need about 4lb of lead to balance their buoyancy when near empty. Steels tanks are often negative when near empty. What tanks do you have?
For BP/W, you and son are likely identical in terms of plate, I think, so I'd just focus on a set for you, try it, then adapt when getting a separate set for him.
I would get your octo the same quality as your main second stage. It will be used in stress, you do not want lower quality adding to the stress.
All the SP regs are good. The hydros BC is thought over priced for what it is. The regs not as much, though something like the Deep6 is very good and less. But a reg from a local shop and locally serviced has extra value as well.
The second stage being adjustable (like the 370) is good. In the first stage, a low pressure port on the end, 5th port, is nice for hose routing but not essential. A turret is also nice but not essential. Sealed is also nice for less maintenance worry. I'm not sure if the mk11 is sealed. If I wanted the best, to me, SP now I'd likely get the Mk19 as it is sealed turret, and a G260 or D240. But the 370 seems to cover the basics as does the Mk11. The Mk2 is dirt simple reliable, but unbalanced so harder to breath as the tank reaches empty.
I think getting some from the shop is a good idea. Do they carry any other brands? Atomic Z2 seems nice among the less expensive that your shop might carry. Within the atomic line, the only differences are in the materials. Which makes little difference.
Best First Stage?
I've got the Deep6 7m suit though have not gotten it wet yet.