You are going the right way with the Sherline, it can't be beat for the price or anywhere near it. It has a 72:1 ratio or thereabouts if I remember correctly, so a very small, fast motor is required.
A 72:1 torque multiplication means even the smallest motor will do, but it needs to be stepping very quickly to get any decent rotation speeds at the chuck, and while big motors may have high initial torque, it falls off quickly as RPM's rise.
A small motor with a flat torque curve is perfect. I use a little 110 oz/in or so PacSci single stack. It has more than enough juice through that kind of gear reduction to spin a moose, and an inductance of only about 1 mh - not a misprint, its not 8 or 18 or 34 or some other number we are used to seeing, but just 1. If you don't know much about the math on how the motors work in practice, lets keep it simple and just say it keeps roughly the same torque it has at speed as it had at rest (holding torque). It will still be turning out 80-90 oz/in at speeds where similar 280 or 495 oz motors are putting out only 30 or >14 oz respectively - the speeds a stepper attached to this kind of reduction will need to spin at.
I don't know what to say about software, I don't do the same kind of stuff you do, but I would say before spending a ton of money on a fancy pendant you should try a simple one first.
I personally love the simple hack of using a simple Xbox controller as a pendant. You can get them used at a game store for like 10 bucks and the Mach3 patch to use it is free off of Artsofts site. Or just use a generic PS2 style PC game controller. Plently of programmable buttons there. I prefer mine over my "proper" pendant actually.