If it helps, I've been working from home with vpn access to my office since 2004. We use IP based phones in the office and I have the softclient on my phone, not sure voip is a big data user, and does not require major high speed lines. In addition we all use Yahoo IM and Voice as well as Xlite connected to SIP Server.
I was 1mb and am now 2mb (free upgrade with BT). I have never had a problem with quality of line. I'm not a big downloader of movies/files etc but do the odd time need to transfer large files (40mb or so) from my PC to and from work. Plenty of ftp, and connections via telnet to Linux boxes.
Since Wed Mar 31 15:04:04 2004 I have used:
[FONT=verdana, arial]Data uploaded[/FONT]19.67 GB [21117931071 bytes] </STRONG>[FONT=verdana, arial]Data downloaded[/FONT]66.46 GB [71362560180 bytes] </STRONG>
Based on that I would at least start with 1MB/20GB offering, sounds good to me...then revisit if you've problems. I don't expect so though.