Sam,
1) With all the extra weight from the angle iron, new z axis and larger motors, and except for lowering the z into the spoil board I have not seen a need for the hold downs. Even though I do plan to do something in that area in the near future.
2) The ballscrew I got off of ebay came with no documentation, I just hooked it up and it worked. There are 5 threads per inch and I don't know how to determine pitch, but it takes a little over 4000 steps per unit in Mach3.
3) And yes it is 1:1 ratio belt drive
4) The quality and price of the steppers is excellent, but in my case if I had to do it again lacking gearboxes, for a little more money purchase a geared motor.
5) Yes the 1 1/2 inch angle iron stiffen the frame alot and I was able to use the original wheels over.
The whole conversion process took about four days to complete, as I also need my machine up and running quickly.
I basically did only as mush as I had to get the increase performance from my Ascension upgrade, this was just a learning process.
By the way thank you for your great pictures, I am in the process of designing my next machine and just might play with that option.
Again anyway I can help just ask, if you want to talk one on one send me your phone # (I have one of those local call anywhere lines) and we can talk.
Tom