I would not buy a 2nd hand Tamron f2.8 28-75mm, especially because it is one of those lenses that is either loved or discarded... you are likely to get one of the soft ones.
Another lens that seems to be gaining some fans lately is the Sigma f2.8 24-70 DG. Again, I would not buy it 2nd hand, especially since its a rather new version... so any 2nd hand ones would be from not so happy customers.
But since you have set your heart on the Canon EF f2.8 24-70 L, you probably will not rest till you have one, so the lens you are getting now may be just intermediate? Since Florida is sunny, maybe it is a good idea to go for something cheap but good, like the Canon EF 28-105 USM II? It will focus faster, if not more accurate. I feel that with all these you do lose out in the wide angle end though... I do not know if you like to take wide shots. And while it is not expensive, the Tokina 12-24 is not really cheap either...
Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30