Ray,
I am pretty much in the same boat. I have a simple system and just want to control it. I have looked at Aprilaire and others but am currently leaning towards Honeywell. I haven't found anything that is direct IP.

Again their system is larger than I need and does have a central control box like the others. But is is fairly small and it looks like it will work with existing wiring. Check out the VisionPro IAQ and W8735A1005 Serial Adapter. The whole setup with second temp sensor can be had on the Internet for around $300.

I'm hoping to get one for Christmas. If I do I will be happy to share the Interface I write.