We have an office of about 30 people, exchange server on premise, data replication (throttled) to another branch, plus that branch (18 people) pulling their emails from exchange, VOIP traffic and then internet access. We started off with a 2mb fibre line and then a year later upgraded to 4mb and then a year later upgraded to 6mb. Voice obviously get priority, but other than that we managed fine. Obviously we would have always wanted more bandwidth but we made do. It never worked on a 4mb ADSL line. It was just too slow. If you can afford it, get the light-speed, otherwise start on the smallest and then upgrade. I'm sure your ISP won't complain about upgrading your line during a contract.