Cambria Bikes is selling the Ultegra rear derailleur for a steal.

For shifters and front derailleur, Tiagra and the best bang for the buck in the lower end.

A cheap low end cassette will do. The difference between low and high end cassettes is mainly in weight. Make sure all your components are for the same number of cogs.

Look at some dual pivot Tektro brakes.

GS and SS are different derailleur cage lengths. A longer cage lets you have a wider gear ratio. Get the long cage version.

Go to your local bike coop and get some free handlebars and $50 dollar wheels.