Piece by piece acquisitiion of a kit is expensive.

If money is tight, as if it never is, then best to find a donor bike where you can strip it of the components and resell the frame and fork. Often you can get free parts that way. Less likely to happen with the top end Campy parts, but easily achieved with mid level Campy and Shimano 600/Ultegra and often Dura Ace too.

Alternatively to satisfy your urge to get it on the road so you can enjoy it, find a complete 105 group here or ebay to install and upgrade over time.

Have fun and happy riding.