Rainbow8 should be very high on your list. In general stay away from Guarder stuff, the fit is just awful. Anything Airsoft Masterpiece, Edge, GunSmith Bros (all same OEM) will be fantastic, CowCow, AIP and DP is also really good.
It's a very deep rabbit hole but incredibly fun.
For the HPA you can either tap your mags (yuk) or look at a Primary Airsoft adapter which is probably the best avenue to go down.