Great question. If your guest room is only used a few times a year, that's some serious square footage to take advantage of. But creating a guest room that coexists happily with a home office can be tricky. It sounds like you're on the right track by thinking of a fold-out bed. When it comes to making one space serve two purposes, especially when one is sleeping, I'm a huge fan of murphy beds. Some units include storage, which could be the perfect place to stash office supplies. While you want this space to work for you, make sure it's still a welcome and relaxing spot for guests. If your out-of-season clothes call the guest room home, make sure there's enough hanging space and an empty drawer or two for your guest.
BHG.com has some great home office slide shows to get you inspired and motivated as you tackle this project. Here are my favorites, along with a $360 home office I designed for a REMODEL Magazine article. That's a perk of transforming a corner into a home office--you get a budget-friendly boost in function! Best of luck--let us know how it goes!
http://www.bhg.com/home-improvement/storage/around-the-house/stylish-smart-home-offices/
http://www.bhg.com/home-improvement/storage/around-the-house/small-space-home-offices/
http://www.bhg.com/decorating/room/home-office/home-office-decor/?page=1
http://www.bhg.com/decorating/room/home-office/thrifty-home-office-ideas/?page=1
http://www.bhg.com/decorating/room/home-office/affordable-home-office-decorating/
