What is the maximum stock height I can handle if I wish to drill and mill threads say M2.5?
After you mill down your base plate. Then you have the option to move the spindle motor to the higher location bolting position.
I’m not sure about the M2.5. I have milled and thread milled 1.25" thread already.
The Z travel is 6 inches. The lower height lets you surface the table. The higher height lets you get to within about 1 to 2 inches of the table depending on your tooling. If you use a vise with parallels the bottom of your stock will probably be about 1-1/4" off of the table.
M2.5 is very small. If your work isn’t too tall you’d probably be able to make any depth of threaded M2.5 hole that your threadmill supported. I would guess most threadmills that small can’t make a hole much over 15mm deep, but all of my thread milling experience is with M5 or larger.