Speed should be fine for VOIP.
I had Vonage for quite a while .. zero issues whatsoever .. might be worth getting the "500 min" plan if it is not going to be used all that much - $14.99 + fee's per month .. (i assume you don't want to get locked into a long-term contract.
I moved form Vonage to Sunrocket a few years ago - again zero problems .. unfortunately - they went bust .. so last year i moved to Viatalk ... which for the most part has been working - however its been MUCH more spotty and troublesome than any other VOIP service i have ever had over the past 4-5 years.
I'm now mainly using a pay as you go provider .... callwithus.com or future-nine.com and viatalk as a backup (as it is not exactly reliable for me)
If i were you - i would stump the few dollars extra for vonage .. it just plain works - its solid - its idiot proof - the control panel is much better than any other out there - and for a new VOIP user its probably all you could ever need. ... if you can find someone on here to give you a referal - you should be able to get your first two months free ...
Xfer of the phone number will probably take a couple of weeks ... make sure you don;t turn off your old service until the xfer has gone through completely.
Yes Bittorent use will effect call quality - unless you have a decent router that does QOS and prioritizes your voice traffic - you may have to limit your upload/download speeds at times when you may be expecting calls - else your call quality can get very choppy ...