Mary's been using a Sennheiser G2-100 UHF headset (when needed) for years.

It's not cheap, but sound men love it. You just plug it in and it works, and the quality is better than the SM58 you'll find in a regular PA rig.

The better radio mic ranges are available in a guitar version. You can't easily combine a guitar with a vocal mic: you have to have two separate radio transmitters and receivers. This is desirable, as then each is a separate channel on the mixer so the sound can be balanced properly.

Anahata