I went with the US3000C as those were included in the kit (and I couldn’t find them cheaper elsewhere). You can pair the US2000C and US3000C, but dont think they are compatible with the US5000.
I’d do the calculations based on price/usable capacity but also consider weight and space requirements. The US3000C is over 30kg
My initial thoughts were to put them underneath the stairs as well. After a conversation with an electrician I changed plans, though due to fire risk concerns. The stairs classify as emergency exit so would need to take extra fireproofing measurements. Also the space underneath the stairs is quite small already so there were concerns regarding adequate ventilation but also ease of access for maintenance/installation (as mentioned above, that stuff is heavy).
I decided to put everything in a spare bedroom which we only use for storage anyway. I put the battery rack on wheels so I can easier remove it in case of a fire.
Voltacon does some nice looking cases for the US2000C as well. In general Voltacon has cheaper batteries (which look suspiciously similar to pylontech). I didn’t choose them
Due to lack of east accessible documentation and also couldn’t find a forum/community.