Mud guards split opinion as they can often lead to trapped wheels if it’s very muddy. The Ass-Saver type are cheap and effective.
Kneepads, try some on at your local shop. Most important thing is that they fit and feel like they’ll stay put when you hit the ground fast at a funny angle, and that they’re comfortable enough to wear for as long as you plan to ride. Good roundup in the Singletrack magazine a couple of months ago.
Shoes, 5:10 Freeriders are the default standard. Depending on size and colour there are some decent deals around £50 floating about. They don’t dry quickly, but if you don’t plan riding more than twice a week it’s probably not something you need to worry about unless you have no heating at home. The Adidas Terrex Trail is getting some love as a good alternative, and can be had for about £60 right now. Regular decent quality Skate Shoes (Vans, Etnies in particular) are a good option for occasional riding and some prefer them. Look for properly skate ones not street shoes – reinforced toes and heels, stiffened midsole, softer sole etc. Again, don’t expect quick drying… A lot of your control and steering when going downhill comes from your heel, so it’s important it’s properly located.