I have an American 2720 so can answer your questions:
The preinstalled US mapping cannot be removed without overwriting it with more US mapping, the European Update DVD will not do this. There are various hacks that might work such as running a US update but pulling the plug in the early stages so that only a small part is installed. Sounds dodgy but I was assured that this does work, you need the US update disk to do this though.
Any European mapping you install will go into the supplementary memory, this is what's left with the US mapping installed. I have v7 NT US mapping on mine and this leaves enough for about half of Europe.
European mapping needs to be installed using Mapsource or the Mapinstall utility.
If the unit doesn't come with legit/non pirated European mapping then you're going to need to buy a full version (not upgrade) which could make it a deal breaker.
I was lucky, mine came with full city nav that the previous owner had bought, however this was a non NT version, when I upgraded Garmin UK reluctantly let me have the NT version which being compressed means more mapping fits onto the unit.
The price from your link is a bargain but only if you can get European mapping included or at a knock down price.
A non navigable base map can be
obtained from here.