I would definitively do the movement…
We did a installation for a customer with more than 600 users with no problem. There is no problem using Asterisk, but if this platform if new to you, I seriously advise to get some help from a Digium Certified professional (dCAP).
For so many users, I would suggest a clustered installation. Also be sure about using separe network for data & voice, VLANs or both. Very important… choose quality materials… and pay special attention to phones quaility (it is better to spent a few dolars more there and got a good phone).
About network bottleneck, you should not be afraid of them as far as you setup correctly VLANs… If you want to use VoIP also for external calls, problems may arise if you do not choose the right WAN links (sDSL / Qos),
Do not heasitate to contact me if you need further info, and good luck !!!