Ah, that's right - All versions of the 125 have 4GB that could (for now) cover Western Europe.
No doubt about it, at the rate the Western Europe map is growing, the only way to avoid taking pieces of it at a time will be more space. The good news is that, if all works as it should, MyTomTom will look at space on your unit and space in any installed uSD card and will let you know what's needed.
My own experience so far has been that it's better to wait until MyTomTom says that a uSD card is needed, and insert it on demand, rather than try to anticipate the requirement and install one in advance. So when it comes time to deal with it, you might consider having the card uninstalled at the time, and poke it in there when requested to do so. Seems to avoid confusing MyTomTom - an application that is pretty easily confused!