Actually Copper can last very long it , and the double protected insulator on the wires are very tough too. For 30 years old house not sure they use double insulator or other method. I know 80s house use lots of double insulator copper cable and like my current renovating house the original copper cable still flexible and look good.

If copper expose to air it become dull because it oxidized, and take long time to turn into black than green.

If your old house have lots of expose copper than 30 years sound correct time to maybe replace. But if all are conceal should be ok.

But for any modern house, i think you find limited socket and lighting points. So that maybe better to rewire entire house.

10k good number to start for single phase.

10k for 3 phase is pushing the limit. 15k for 3 phases is better. IS better to plan more than worry not enough.

If the mixer distance is align properly should not be hard. What is the conceal connector made of? PVC or metal like copper / s.steel?

Why need glue, later when change is not easy.

The water heater plumbing is cheap. Mine I pay 800 per toilet, for ground floor more expensive.

Mine also same, if use tap you need to pay much more for taps.

Make sure they give stainless steel angle on the wall.

My entire piping include relocating wc and rain piping about 11k.

Yup including other works too.

Think OK. Sometime cheapest may not be the best. As long you feel comfortable dealing with him, and not too expensive.

Use SS piping, footprint will be smaller because SS are tougher and stronger for the same diameter and thickness.

It take so many decade to corrode the SS, by than the house will do another reno.

psf get the quote from hardware shop, they will know. If you ask plumber it will be more expensive, is material + labor + fitting + expenses.

Fitting is not cheap.