I'm not a fan of having the prerotator unit mounted on the mast. It seems like an un-necessary complication to have a drive shaft for an ordinary prerotator. I'm always for the smallest lightest simplest solution. Most everything I do with gyros is toward the ultralight side.
A PWM speed control is, in my opinion, the best option for soft start on an electric pre. Coiled wires are simpler and cheaper, but don't seem like a realistic choice for batteries other than lead acid. I've switched to LiFe batteries and am happy with them so far. I haven't gone through a winter with them yet. I'm told the cold weather starting performance is poor, but that if you pay attention to battery temp it can be worked around.
Both spur and planetary gears provide high efficiency reduction so I like them both, spur gears are easier to source and have a fuller range of ratios. Planetaries are compact and strong.
I've been working with brushless motors lately. They are cheap, compact, and powerful. There is a nearly endless variety of them available. the ESC's are also comparatively plentiful and inexpensive. Drawbacks are they don't have as much torque at slow speeds, and sometimes "cog" unless they are sensored, and they tend to run at higher peak rpms which require greater reductions. I don't mind patting the blades once or twice to make the start up easier but some won't want to. I've been thinking about how to safely bring blades up to speed from a standstill with a brushless system that won't burn up the motor or controller but it's down the list of priorities right now.
Brushed motors have great low end torque, are more tolerant of the type of duty cycle spinning up blades is, and are generally inexpensive, but really good ones go up in price fast and the controllers seem to cost more.
If you're aiming at making something for the general gyro market than I think the ability to easily start up from a dead stop and driving the blades up to max rpm at a standstill on the runway suggests your on the right track with a good 12 volt brushed starter motor with integral gear reduction. Most bendix/ ring gears in use today are nearly or exactly the same ratio (Wunderlich) so optimizing the first stage of reduction should be straight forward.