For a single location,on a single building, there is no much justification for going VOIP.
If you had several locations, some of then overseas, with a data network already in place that you could use for “free” calls, that will be diferent.
But with the low prices on new and used phone equipment, and a surplus of phone technicians, will be hard to beat the price of a PBX with a couple of PRI links to the outside world.
Just my $0.02.
P.S. The use of a gateway, located on the front of the PBX to make long distance calls at a low price is something that you may want to look into.