Definitely go with steel. You'll want multiple sets of sealed bearings as replacements. The sand is going to be your biggest concern as far as bike longevity. It will get into everything. chains will wear far faster, too. Make sure to clean them all the time. Rust will not be an issue over a two year period. Road salt is the number one rust worry for bikes. I don't think you'll have an issue with that. I would recommend a surly crosscheck frame.