I'll start of with some thoughts on this.
Drilling forces should be pretty low in PVC, and given that you'r doing this by hand just now, accuracy doesn't need to be the top of the priority list?

First idea -
To keep costs low, I'm thinking a long bench, and use some standard steel/alu for rails for a moving head to run on, driven by either a rack and pinion set-up, or for more accuracy, a timing belt (single length, anchored at each end, with it looped up and around drive/idler pulleys on the head). Other option is ball screws, but I'm guessing 6m+ ones may be quite expensive..
As you'll only be dealing with round pipe, you only need an x-axis.
Then have a rotary table at the end of the bench, which clamps the pipe, and some form of adjustable guide on the head itself to support the pipe during drilling/slotting, yet still allow the pipe to pass through/spin.

second idea -
Have the machining head static, and then have the pipe pass through it, however it would probably take a fair bit of experimenting to design some form of reliable feeding mechanism.