Hi there,
Not to complicate your search ... but it sounds like you will be 4 using that car, with luggage to last you a month (so I would imagine 4 suitcases).
Be aware that an "economy car" in Europe is likely to be a VW Polo or something tiny in any case ... so while price is important, I would also check what kind of car you would be getting.
Cheers,
GenevaFlyer