Here's my 2 cents:
The diamond bit is great once you get used to your lathe. I wouldn't buy it right off because sometimes you have the odd "accident" while cutting motors. Once you're real smooth with the lathe, it's a great thing to have - never gets dull and the finish on the comm is great.
Dyno-wise, it depends what you want to do. If you're looking for a tool to break in motors and so forth, the Trinity dyno will work. I would also look at the MuchMore Motor Master. It works well too for that sort of thing.
Dyno-tuning is a whole different can of worms. You can't do much with any of the free-wheeling dynos that don't have any load on the motor. That's a completely different thread though, so I'll leave it at that.
T-Bang Hobby Technology Inc. - www.t-bang.ca
Extreme Performance RC Motors and Racing Supplies - Vancouver, Canada
Team Corally - Trinity - Dekelz.com