Make sure you get it direct from EE (EE-GB Variant), as the unlocked CV-GB variant does not include a wifi calling option. You also must use a 4G EE SIM as Orange or T-Mobile 3G SIMs do not work with wifi calling (the option does not appear).
The quality seemed fine to me, the dial tone was the same as an international tone though, which was odd. It definitely worked when I had no mobile signal.
The 640 isn't much of a step back, apart from the camera, which isn't great in low light.
Check out a shootout I posted here (also against the 535 and 735).
The glance screen isn't quite as detailed as it is on the 830, it can't show calendar text, or integrate with apps (such as MSN Weather).
The battery life is significantly better though, the 640 lasted approx 12 hours longer than my 830 in my tests.
Currys PC World are currently selling open-box Lumia 830's on ebay for £150. These are Carphone Warehouse stock which was returned within the 14-day period, and are almost always in brand-new condition. The manufacturer warranty is valid with them too.