CNC Lathe

Hi, 
im new to this forum, so a short introduction: My name is Marc, i am a mechanical engineering student from germany and as a hobby i am involved in all sorts of machining, 3d printing electrical work etc. I guess the cool kids would call it "making". I would now like to convert my freshly rebuilt Lathe to (Linux) CNC. However i am a bit overwhelmed by the hardware choices, maybe you can give some advice. What i am planning to do:
- Close loop with stepper motors and linear scales on z and x
- 2 hand wheel encoders for semi-manual operation + spindle encoder
- I have a VFD for the spindle (danfoss)
- one extra stepper for the spindle -> normally not connected to the spindle, if i want to do very precise spindle positioning
- Old pc and touch screen for the control
- some manual programmable switches would be nice
My main concerns are:
- spindle encoder -> should be high resolution for threading/positioning. How would this perform at high rpms ? would it make sense to use two encoders, a crude one for high rpm and the fine one for positioning ? 
- Interfacing of the glass scales ( i have some electronics experience, so getting a pulse signal out of them should not be the problem)

It would be nice to keep it somewhat cheap eg. keep the encoders in a range where i don´t need extra interfaces to read them out etc.
My guess would be simple stepper drivers (DM556 ?) + some sort of breakout board <- here i don´t know what to pick.

PS: with close loop i mean between the stepper and the glass scales, not within the stepper

Hi Marc! I would recommend you to use MESA 7i76e, and skip linear scales. You can add them later. One encoder for spindle will be fine. If you are going to use mesa card, high resolution encoder will be fine 400-2000ppr.

Thanks for the recommendation. The close loop with the glass scales is a must for the precision i want to achieve, speed, or fast rapids is not really important for me. wouldn´t a 2000ppr give a 6Mhz signal at 3000rpm, can the card handle that? Also the mesa 7i76e seems quite expensive, not sure i neet that many inputs, also i only need 2 axis (3 withs spindle stepper)

Note that the 7i76e has only one highspeed encoder input and two low resolution MPG inputs. Since you need more encoder inputs you would need to add a 7i85, 7i85S or 7i89 to the 7i76e. I use a 7i85 with a 7i76e for my mill with steppers and glass scale feedback.
As for the maximum pulse frequency you might want to wait for PCW (mesa) to answer that.

Sorry for wrong information, I had servo motor with 2500ppr running on 3000rpm, but i used electronic gear reduction 1/8.

How well do those parallel port breakout boards work? I guess the sample rate would be much lower than the mesa boards but maybe good enough for ->low speed high resolution ->high speed low resolution ? is such a configuration possible in linux cnc? From what i read online the parallel ports solution seems to be harder to work with. I wouldn´t mind some extra efford as i have more time than money right now, but if it would be a nightmare i am ready to spend some more money.

Damn those specifications do look tasty. I would need to expand my budget quite a bit. Would there be any advantages (except beeing plug and play) performance wise? I mean the most demanding things i plan to do is cutting tapers or threads.