Z-Wave is certainly not a cheap choice, but effectively one is confronted with the problem of needing to use protocols which the products you intended to use communicate with. This is a good reason for multiple automation networks, as different communication needs can be handled appropriately to the budget, requirements, etc.
For example… One-Wire is a cheap and easy way to run a bunch of wired sensors around a resonably small area. It is a daisy-chained network with basic (cheap) cable, so you don’t get into the high cost of running many pairs of more expensive copper wire back to a central location. The devices are also cheap, and have been around for quite a long time. (there is a ready-to-use 1-Wire/Ethernet gateway which works with OpenHAB)
There is also 433MHz which can do A LOT on a budget, but if you want to avoid batteries then wiring would be best. Honestly, I tend to agree about avoiding batteries, but sadly there aren’t many products out there that you can easily externally power (and they’re mostly wireless).
You mentioned the hub, sensors, and gateway… for me, from an architectural perspective, it is important to also keep in mind the connectivity (network, protocols, transport) and network interface. The choice of sensors/actuators, network, interface, gateway, etc will all be dependent on your needs and how you architect your solution. MQTT is TCP/IP level, so once you have a network interface from whatever network to OpenHAB, you can put everything on MQTT (I do this with OpenHAB + RFXCom binding + MQTT binding). Or you can use MQTT directly from something like an Arduino.
Regarding your desire to use PoE… I too have a PoE switch… and would love EVERYTHING to run on PoE (for many reasons). However there are some reasons why this is not feasible for cheap sensors as the electronics required for appropriate power conditioning are apparently not cheap. There is a guy called Jonathan Oxer from Superhouse TV and Freetronics who has designed a custom Arduino which has PoE (and Ethernet obviously) built in for much less than adding a PoE shield to an Arduino. He has also wired his house almost entirely with Ethernet, PoE, and Arduinos to control everything.