Scook, forget the helmet bit, go with what you feel best in that area, as for your OP in Oct08 I owned nothing that was cycling specific, so top down:-
helmet - I have one but have taken to not wearing it, personal preference
headwear - little wooly hat can help on cold days, apart from that, I need the relief from overheating
jacket - Winter/waterprooof I highly recomend the
Altura Nightvision(I paid £50), but it is warm. For lighter wind/water repellant I've bought the Monatane velo featherlight in jacket and gilet versions(£55 the pair if you shop around). If you really want to stay dry in warmer conditions then a bin bag with holes ripped for arms would do it, for pennies.
mid layer/outer shell - I'm a big fan of merino wool stuff, but it can cost silly money, micro fleeces serve the same sort of function. I have merino from Rapha(£125) and Micro from Craghopper(£35 for 2), both do the job but at very different price levels.
jerseys - only got one so far and seems nice but pretty tight fit despite being XXL, will know more as Summer approaches.
base - have 3 of the Rapha merino t-shirts and, compared to cotton t's or fancy synthetics these are the dogs danglies. Had to phsyche myself up to pay £90 for 3 t-shirts but I'm glad I did. I don't worry re washing, go in with the rest at 30deg and no probs after 20 washes.
shorts/longs - started out in boxers and jogging bottoms, moved to cycle shorts waist padded(£20 each) plus jogging bottoms, then cycle shorts plus baggy shorts......and so on. Have just taken receipt of first bibbed 3/4's with padding(£35 DHB from
Wiggle) and seriously think waist stuff may be a thing of the past. But this will also be impacted by personal physique, skinny folks can wear what they want
socks - serious thought here, cold feet have caused me to consider some of the ridiculous prices some places charge for socks.
shoes - I'm still a flat pedal person so very limited knowledge, but have just taken delivery of a pair of Impact Five-Ten Lows(£75). Only worn them in the house tonight but very comfy and you feel velcroed to the floor.
Hope this helps a bit Scook....cheers Al