An alternative connection is to buy the Starcom Bluetooth module, which will happily plug into an Autocom. Ive read on this site (but not tried for myself) that this works fine.
Its a lot more expensive than the Autcom wire (£70 I think) but it does get rid of the GPS to Autocom wire, which in my past experience with Garmin has broken several times with very annoying results.
I've had the Garmin Quest (v1) myself a few years ago. Its a perfectly competent unit which has the benefits of the Garmin interface which allows you to pick the back roads more easily, and the Mapsource software which allows you to pick the back roads more easily whilst at home... Minus side its got a very small screen, it can lose its signal under a cloud (honestly!) and I, personally found it very slow at displaying complex junctions.
See if you cant get a demo of them both before you make your mind up, as I think they both look good on paper, but are very different when it comes to using them daily.
Cheers,
Mike