Actually, the main benefits from a true variable speed are comfort and
noise reduction. Energy saving is just a fringe benefit. What you are
wanting to do is certainly possible. You could retrofit a Model T engine
to electronic fuel injection with enough time and money. I seriously
doubt you are going to find an ECM for any where near $200 though. Fwiw,
we have installed a lot of variable speed furnaces and air handlers
(mostly Trane, Amana and Carrier) and I have not heard of any problems
with electrical noise at all. Good luck with your project if you go
thru with it. Larry