Mmm, if you start out with low res sources you're not going to be able to do much with them. Kinda screwed in that department unless you can get some decent res sources although I have found that plotters/large format can be more forgiving with lower res stuff.
In any case mmm, since you're designing for A2 in photoshop, create your canvas @ 17"x24" or whatever and 300dpi or higher (300 is pretty standard).