You can get an S10 with filter and carry bag for £20-25 if you're a bit patient on fleabay. If you're interested PM me for deets of a bloke who sells replacement ballistic lenses for them - it's a piece of piss to change them over and the lenses are great. Just as well cos they're about £15 delivered IIRC.
You can play in them. With an unmodified filter in, you have to do as they do in the RS world, ie take long slow deep breaths: obviously your instinct will be to breath faster as you run around, you just have to remember that you'll get more oxygen into your system by breathing in the recommended way than trying to gasp and stopping cos you can't suck that much air, then trying again, and again, ad nauseam... I've found that they don't fog up significantly and I can fog normal specs while doing nothing more vigorous than a quick walk! I'm pretty sure it's the way the air is designed to flow inside the mask which prevents fogging, but I can put you onto a Russian Anti-Fogging solution for £1.50 that takes care of the slight bit.
With no filter in they don't impede your breathing at all. You can see out of them fine too. The SAS wear extremely dark navy blue one-piece jump suits as well as black ones. I can put you onto an assault hood for over the S10 also: i'll have to look it up though. They carry MP5's so you'll want a vest with 3 pouches for those mags on the right and a cross draw holster on the left; shotgun cartridge pouch or loops above right, grenades upper left; radio top left. Drop-leg MP5 mag pouch on left leg. Maybe some extra grenade pouches on the back of the vest over your kidneys, or on a drop-leg molle platform on right leg. Pistol mag pouches on your duty belt.
AFAIK troopers can choose their own pistols, so in theory you could choose anything, but if you want authenticity, I'd stick to one of P226, Beretta 92 FS / M92, or Browning HiPower. They also commonly use SPAS 12 shotguns, so you might want a sleeve/holstewr for one of them on your back & oh yeah, check Action Hobbies for the recently cheaperestified listing of a decent SPAS 12 RIF springer...