perfect...so the "rear" problem is solved...XT cassette is okay also for 105 hub...nevertheless for more robustness I think about mounting 135mm hubs...
also the XT rear derailleur 9sp is okay with the 105 5703 STI levers...
105 levers are okay with a possible mounting of the Avid's BB7...perfect also this...

now...help me solve the "front" problem...how to combine the 105 5703 STI levers with the XT 48/36/26 crankset?! which front derailleur?!