Apparently there are over 130 diseases that can be transferred by using shared headsets. It might be worth investing in one from the start.
My first headset was the cheapest I could find at the local pilot shop in Kissimmee (USA). It served me well and is now my backup/passenger headset. I recently bought a SkyLite SL-900 over the internet. There's a guy on eBay who sells these quite cheaply: I paid something like 120 euros including a headset bag, shipping, handling, taxes and everything. Very happy with that one.
http://www.flyskylite.com/products/sl900.asp
I'm currently contemplating fitting one of these aftermarket ANR kits to one one of them. They got quite good reviews in the latest Pilot magazine, but the SL-900 is not on the list of supported headsets, and they don't respond to enquiries about them.
Edited to say that it wasn't Pilot, but Flyer Magazine, Jan 2008, which did the review.