I would suggest a DECT 6.0 or a 5.8ghz phone would do the trick. Both operate in spectrum's where interference should not be an issue.
As far as range, depends on what obsticles your have, walls, trees, and most of all distance.
Some business phones here (expensive) AT&T phone has a range extender if you need it but generally over kill.
Consumer grade is usually fine for most users.
You could look for an antenna on a Corded Base model that may help range.
I have a Panasonic DECT 6.0 and range and sound quality is great so far.