Since you’ll be using MapSource it’s going to be a little bit simple 
Download all the countries you’re going to visit (get the mapsource installer version). Install all those on you system without uninstalling the previous maps. You will now have a directory that contains all the tiles needed. Now you only need to create the overview file (.tdb) to make all the tiles visible at once in MapSource. For this you can use MapSetToolkit, Mkgmap or cgpsmapper.
I’m not running Windows here, so I can’t provide you with a quick step-by-step guide for MapSetToolkit, but it seems to me that using MapSetToolkit will be easiest and quite doable.
PS. I’m deliberately not stating the maximum number of allowed tiles, nor a counter to keep track how many tiles are selected. I’ve had many people scraping the site without good reason which caused overloading. I’m very sorry I had to implement the max. tile limit which is causing you problems but without it this service would simply desintegrate.