Few questions from a first time buyer

Hey guys, just had a few questions if you don't mind. I'm a total newbie to this but I played around with a friends Foxtrot Mike FM9 at the range today and really liked it.

I'm looking to get my own and I was looking on Foxtrots store but I was a little confused at buying an upper+lower being a lot cheaper than the full gun.

https://store.fm-products.com/products/fm-products-m9-sp-u-5-rc-5397 - Upper
https://store.fm-products.com/products/fm-products-m9-a-lwr-comp-rmb-5397 - Lower

Is this all I would need to get started? Or am I missing something here?

Also as far as stocks/braces and suppressors go, is there any certain attachment or fitting (idk how to word it) that I should be looking for to make sure they're compatible?

Appreciate any help