At least here in the U.S., one can buy ready-mixed grout in several different colors or shades and one can also buy grout that's dry in a bag to mix. Some experienced people prefer the mix it yourself type, but I find the ready-mix handier and easier to use. I wouldn't worry about the mixing ratios if you choose to mix it yourself, though. The instructions are quite clear. Just mix small batches to start. That way you can add more water or grout if needed.
You're correct, most grouts require sealing after curing for at least twenty four hours. It's usually brushed on, but can also be purchased in spray on cans.
Be sure to completely remove any discolored or damaged grout before starting the replacement and be double sure to clean up as you go along. Dried grout can be a real bear to remove and only gets worse as time goes on...
Best of luck!