How 'bout a Cirrus SR20 or SR22? Apparently they're Part 135 ready
"out of the box." Any opinions on these?

Better check on the single pilot aspect of it. For IFR, I think you need
two pilots or a 3 axis autopilot that is part 135 approved. Last time I
looked a few months ago, the Cirrus only comes with an S-TEC autopilot.