From Sora to Ultegra, I'd go when I had the whole group.

If 9-sp is your yen, Jim is right, you can get a complete 9-sp STI group for about double what you were going to spend on the Ultegra FD/RD. I just bought a complete Ultegra 9-sp group, front to rear, chain, cables, cassette, etc, $325.

If 8-sp is the deal, the 3x8 Sora's will work fine in double, you can adjust to noise-less trim on the FD. You can use 600 tricolor FD/RD and be shifting smoothly for $50 or so. 105 will work fine, but 8-sp 105 is hard to find.

As far as donor bikes, there were a lot of Cannondales put out with 105 STI stuff, you may be able to pick up an entire bike, STI-equipped, for $225-$275.