I'll assume you want windows Pc's and stuff....
winproxy sucks, all crappy software like that does... lacks the ability to do what a real proxy should.
get a soho sonicwall and a cisco 24port switch... 3524 is reasonable, at least get something managed. for a mail server use what your famailiar with, exchange and lotus notes are feature filled, postfix is alot cheaper (read free) , but lacks collabrative features.
for the most part dont buy and deal with crappy hardware, the cheapest part in IT is the hardware, buy quality stuff so you dont have to worry about it being flakey, or if soemthing does have a bug you'll know the manufacturer will stand behind and fix it.
Dont skimp on the cabling either, it can be the biggest pain in your ass when trying to track down "weird" issues later, expect to pay ~$100/drop and have them certify it and print you out proof of its cert.
Buy computers with onsite support... Dell is pretty reasonable in this area, and you dont have to worry about getting parts and crap or swapping them out, they'll send someone to do that and verify functionality.