I'll touch on a few things.
FIrst off about the undershirt, talk to your association, they may require white OR black. We wear black.
For a hat try to get fitted, it looks better(or so they say).
I own both Cliff Kleen and Honigs striped shirts and they are both fine for me. I don't really notice a difference but I don't own any of the promesh shirts. One thing is you want to make sure that your association allows promesh shirts.
As for knickers I only have cliff kleen so I'm not of much help on that. I also have shorts by both companies but the honigs shorts seem to be thicker and harder to see the stripes show through.
Spotbilt does make football shoes, check out
http://www.honigs.com When looking for shoes make sure its comfortable, all black(if the logo is white its okay you can dye it) and its really personal preference from there.
Yeap, socks are one piece.
Basically talk to people from your association, ask the head guy about what he recommends for you to get and what the association is or isn't allowed to wear. Some can be stricter than others.
Good luck. Hope I helped a little bit.