A hybrid system, which is part PBX and part VoIP is a good approach, but this also requires that you still know how to manage both systems. It is an intermediate step to full VoIP. When you look at implementing VoIP, you will want to consider a few things.
You may have to upgrade your cable plant, routers, switches, etc. For some connections that require analog, your switch manufacturer can supply you with an analog card or you can use the telephony part of the switch. You will want to have layer 3 routing on your network (ideally), but in smaller environments you can add VLANs. You will need to check into a bandwidth manager to assure that your voice packets receive the highest priority.
In addition, you will want to be sure that your network is healthy. Your cable plant is a big area to check as well as your active electronics on the infrastructure. They should be running healthy and error free. Many of the VoIP manufacturers will audit the active electronics, but you also want to check your cabling to be sure that it tests to category 5e specification. If it is faulty you will have a slew of problems.
You will also want to run your own cost models to determine what type of phone to use (the more expensive one without a switch and the less expensive one with a switch). Depending on what type of electronics you have, it may be much less expensive to keep the switch in the closet. Make sure it has sufficient power backup or your phones won’t work in a power failure. You can use Power over Ethernet (PoE) to accomplish powering the phones over the data cable.