Finish: Your paper finish will affect how you're able to print and how much ink your invitation will take. Glossy papers take a lot more ink and longer to dry. Linen papers and papers with more texture have a much higher tendency to bleed, and so they won't work with some at_home printers where you can't change your ink settings. If you're not interested in researching finishes because you're not a weird nerd (like myself) who likes to feel on paper, a matte card stock is an easy, relatively foolproof option that will more than likely work for the printer you're using.
ADDRESSES: You can choose to address your invitations by hand, or you can decide to print labels. Don't let the WIC bully you into thinking your own handwriting isn't good enough, or that the convenience of labels is tacky. As for the actual addresses, here are a few tips on how to address invitations formally, accounting for a whole host of last name conventions.