A sensor in the group is a thermally a million miles away from the heating element in an HX. One might do better to model group thermal performance and install sensors for ambient temperature, air flow and direction - all of which impact an e61. Back in '08 I measured a VBM HX w PID with a Scace 1

I have definitely been looking at that one. The top/handle worries me a little. I've heard it can fall off too much

Kingrinder at $100 or so is another option. Looks like Amazon US has the K6 at $100 after a coupon right now