You certainly don't need a zoom to hone your technique. Many of the older members of this community will have started their SLR ownership with one or two primes back in the days when zooms were optically rather worse than they are today.
I don't have any of the lenses on your shortlist (and I think you meant the Sigma 30mm f/1.4 as I can't find a Sigma 30mm f/1.8 on the Sigma site). If you haven't seen them yet there are reviews of all three lenses at PhotoZone here
Is there any particular reason, such as Depth of Field perhaps, that you are looking for a faster prime rather than the EF-S 17-85mm f4~5.6 IS USM (check Gordon's review here
) which is sold as the kit lens here in Europe? The price of the 40D with this kit lens is cheaper than buying a body and the EF 28mm f1.8 USM separately.