Some points
The artificer can turn any heavy armor into power armor.
It does. I'm not sure that is indented though, and I think that initial power needs adjusting a little, since you can put on any armour and turn it into power armour in 6 seconds. Even Tony Stark needed weeks in a cave!
It should probably be comparable to creating a steel defender - it takes a long rest to build one, but you start with no "base item", you create the whole thing from scratch.
Rather than an add on, Guardian and Infiltrator Armour should be there own armour types:
Guardian: AC 18, no strength requirement, stealth disadvantage, 30lb, special
Infiltrator: AC 13+dex mod, no strength requirement, stealth advantage*, 10 lb, special
*it needs a name-appropriate buff!
I'm a little shocked there isn't more discussion about the force field temp hp.
It's basically an unlimited uses Second Wind. I agree, very powerful, possibly overpowered. Definitely overpowered compared to the other armour option.