Were it me, I wouldn't consider more than 1/4 horsepower for that - it's simply not needed. The spindle on that little guy is tiny (1/2"?) and any more power could conceivably damage it.
This little guy would be an ideal candidate for a cheap DC motor from a treadmill or the like, as it would give variable speed with no need for belt changes or aught else. This being Spring, i'd be hunting the yard sales/side of the road on trash day for a treadmill with a DC motor and scavenging it for the drive system.
I will say, your abilities with wood turning are superb - bravo.