I would go to Plas Y Brenin. The best guides in the country work there and that's where they write the sylabus. I did my training with one independant provider who was absolutely terrible and I have since heard other peoples complaints about them. I am unwilling to give their name in public however email me at email@example.com and I will let you know. I am not saying all independant providers are bad but if you want a guarnteed decent course head to the Brenin.
I have done the horse shoe in the winter. If there is snow up there I would bring ice axe, crampons, sunglasses and usual winter attire. If you are planning on going over Crib Goch then I would also bring a rope and a selection of nuts and hexes.
Lost Earth Adventures offer guided walks across the country. We cater for everyone, no mater what you'd like to do. Have a look at what we offer and do get in touch if your interested. http://www.lostearthadventures.co.uk/
I spend a lot of time in the jungle and swamps and the only stuff I use is DEET. I use it for months at a time with no problems. At night I sleep under a mosquito net and during the day I use long sleeved shirts and trousers with my trouser legs tucked into my socks. In highly infested areas I sometimes use a mosquito head net which looks like a bee keepers head net. Quite ridiculous and a bit annoying but there is little else you can do.