I’m no pro (by any stretch of the imagination) but from what I’ve heard and know on the subject your data switches are going to be important.
Proiority needs to be given to VOIP protocols on the network so that in busy periods the call quality won’t degrade.
You should also look into a SIPS backup for failover in the event of the internet not being available (if your on JANET in the UK this is vital as their network at risk time is 8.30am tues when you would expect business calls to start coming in and going out).
Cost and feature set of handsets is also important.
As with usual data there is the 90-100m limit on data cable length (ethernet). This limit may effect alot of users who had a phone on a cable further from the switches than this as this limit doesn’t effect normal phones but will effect VOIP.
The other things to consider are the usual suspects: are you buying from reputable dealer? Is the dealer financially stable? What support contract will you get? Can you get training on the system included in the price?
As far as I know these are the basics to research and check up on. If anyone has any more please add as I would be interested to know myself.