Can someone recommend a GPS unit I can walk around Europe with?

I’m new to GPS and have not owned one before.
I’m taking a trip to Europe and would like to find a unit that would fill two purposes.
1) Getting me where I need to go in London,, Paris, Dublin, Cork, Scottland, Iceland
2) A unit that would still be useful for driving directions in my car when I return home to the U.S.

Can anyone recommend a reasonably priced unit that would fill both needs.
Also, do these things come with road maps of all these places built in or would I have to buy something to download or install for European maps?