I'm sure someone with more expertise will chime in but here's my .02... single handed casting preferences don't apply to Spey casting, or at least very little.
A good Spey rod carries the energy with a little cushion of delay followed by a strong recoil to throw the d-loops into position for the next move. Of course the line style you use affects the rod preference - for instance fishing floating double tapers verses compact Spey heads. But for the most part the trick is to get a rod that really loads up and really unloads too. Too soft and it won't recoil; too stiff and it won't load in the same cadence as the long drawn out motions of the Spey cast. You move a lot of line around with these simple but effective Spey motions.
If I may make this suggestion, cast as many rods as you can, get opinions from the great people here and on Dana's site, and make an informed decision based on experience.