Choose the printer, then design. Chances are, each printer can provide you a list (and samples) of digital paper stocks they commonly run that work well for their particular presses. These may be different from printer to printer so it is important to know what stock works well for your printer.
What type of color reproduction can you expect? Can your printer provide you with a digital chart or samples that indicate how PMS colors will print digitally once processed out?
What are the font limitations? Depending on your printer’s device capabilities, they might run into issues when printing small fonts.
Choose the paper. Ask the printer for digital paper samples ahead of time. When you spec paper, don’t think in terms of #3’s or #2 grades, think in terms of digital grades. Your printer should be able to provide you with digital paper samples of various finishes, weights and sizes.
Estimate before designing. If you have a rough idea of your project (size, quantity, color, etc) talk to a printer and have them give you a rough estimate for how it will be produced. Is it more efficient for them to produce the project digitally or offset? Do they recommend any sizes that are close to your estimate but offer greater efficiencies?
Find the grain. In your printer’s estimate, which direction will the grain run? Paper that folds against the grain will crack easier than paper that folds with the grain direction. The ideal grain direction should be parallel to your fold.