How do you setup wifi networking for a multi-floor startup

I am posting on behalf of a friend of mine, who runs a biggish startup.

We are a 50-member startup based out of India and by some combination of bad luck+ bootstrapped funding + Indian real estate demand, have 3 floors of a building separated by 2 floors in between. Now we do have ethernet wiring, but we are having trouble with ethernet due to various issues (next door construction cut our cables, rats, etc.). So we want to move to a pure wifi solution (with some amount of cabling of course).

I'm trying to figure out how to set up wireless so that we can have a single, seamless wireless network between all the floors. I mainly want to safeguard against everyone accidental overloading a single router and have a single ssid+password.

Be warned that it is very likely going to be DIY because of availability of vendors/hardware, so it would be great if there are solutions which can get away with off-the-shelf hardware.

I am hoping that silicon valley has almost entirely moved to wifi because nobody wants to be caught dead working on desktops! Hopefully, other startups had similar growth pangs and could share some pointers.

thanks!