Hi Dave and welcome to CoffeeSnobs....
Ill answer your questions as best I can.
Well mine is 3/8 between the machine and the water softener and then a standard 1/2 inch to the supply...... I personally wouldnt go as small as 1/4 inch but it might work...You will restrict the water flow with 1/4 - and that would make the pump work harder.Whats the right inside diameter thickness for the water supply lead? It looks like the unit accepts 3/8" connector, but the standard wall box valves for refrigerators and those types of accessory water lines is 1/4". Will I hurt the unit by using a small diameter tubing such as 1/4"?
Yep between about 3 bar and should not exceed 5.5 bar (1 bar = 14 psi ) so 80 psi is just over the upper limit - again you might get away with it - just (keep your fingers crossed)Is there a certain inlet pressure requirement? Doing a search on water supply for M20 showed where someone mentioned needing some sort of pressure restrictor. My house water pressure is probably roughly around 80psi.
The drain should be normal plumbing pipes... which can withstand boiling water. PVC plumbing is fine - but not the thin walled drainage pipe. I have used all rigid piping - if you intend to use flexible then yes, you would need to get high temp hose.The drain tube appears to be a high-temperature-resistant, industrial strength tube. Is there a specific heat requirement for the drain lines?
Nope - no need for hot water.* Am I correct in assuming that a cold line feed is all thats needed? No need to send hot water to the unit, is there?
Ensure you have adequate current available from the mains - even the single group uses quite a bit of power.Any other pointers or advice would be greatly appreciated. Plus, if someone had a PDF of the owners manual or service manual, Id love to get a copy.
I have two manuals for other models (multi group and more recent)..... None seem to be available for this model. I can email you those if you wish, the models are all very similar. PM me your email address if you want them.
Good luck with your M20. They are a great machine and make excellent espresso.