The shift/brake levers are probably going to be your most expensive part and will determine some of your other components. I would start there. If you can, test ride a bike with Sram and Shimano (and possible campy if you can find one) levers and figure out which you like best. The cheapest places for new complete groups are usually british sites like probikekit and ribble. GVH is also nice because it allows you build kits ala carte. If you just want cheap brifters check out microshift which are available as the house brands at performance bike and nashbar. There are reviews on this forums if you search. Most report them being functional but with a cheaper feel than shimano. The good news is that they are shimano compatible so there are lots of part options and they can be upgraded later with out having to re-buy other parts.