First off, hello and welcome to the wonderful world of wondering where the fuck all your money has gone airsoft!
There's plenty of trousers with no hard knee armour such as Helikon SFU https://www.military1st.co.uk/clothing/trousers/sfu which can also take a soft knee pad insert (I have some of these myself and they're great)
Best way for carrying a rifle and sidearm is a sling and a holster. Depending on your preference slings can come in single, two or three point flavours (I'd suggest a single point for CQB and a two point for running around the woods). As for holsters I'd suggest you stay away from "drop leg" styles as most people seem incapable of actually putting them on correctly. Try a mid ride belt holster.
As far as the "full head" protection goes, either a FAST style helmet with goggles and a half face mask or something like the WarQ mask would work.