Hi martigd,
The mattress shopping tutorial here would be the first place I would start and has all the basic information, steps, and guidelines that can help you make the best possible choices … and know how and why to avoid the worst ones.
I don’t know what your budget range is but I would consider some kind of “basic” mattress using the materials or components you prefer in a “medium” firmness range which would be suitable for a wider range of people than a firmer or softer choice and because a guest bed doesn’t get used as much the durability of the materials won’t be as big an issue as it would if it was being used every day. You could also choose a mattress that was a little firmer and then purchase a topper so you could offer your guests a softer and a firmer choice (with and without the topper) depending on your guest’s preferences.
If you are comfortable with an online purchase then the tutorial post includes several links to some of the better online options I’m aware of.
If you are more comfortable testing a mattress in person then post #2 here includes the better options and possibilities I’m aware of in the Minneapolis/St Paul area.
Phoenix