Hi berfles,
In addition to jankdc’s very helpful comments and links … I would also make sure you read he tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices … and know how and why to avoid the worst ones (such as the major brands) which use lower quality materials in every budget range.
Higher quality/density materials will also be more durable but will also be more costly and for a mattress that is only for shorter term use this may be less important to you.
Phoenix