I am not a sifu, but I recently did a renovation so I could share some personal experience.

I can't answer your first question since I already decided on Quartz since the beginning, so I've never made any research on solid surface. On % of quartz, if you buy standard one, you most probably wouldn't know. Expensive ones might tell you the %.

Tiles, ceramic is good enough for wall, porcelain is better for floor, but thick ceramic should be fine too if you like the design.

I think you drawing is good enough for KC company to start designing. Maybe you would want to specify base unit height that suits your own height. If you already have in mind how do you want to arrange your tableware, cookware, dinnerware, utensils & etc, maybe you can specify them in your design.

And I would highly recommend Cubix KC if you're into Shaker style cabinet with plywood carcass.

And don't forget to buy a water level and check the straightness of your walls before engaging a KC contractor. I learned that lesson the hard way. 

I think you need to have wider gap between your sink area and hob area to facilitate the movement to the door. From the diagram it looks small. Do not max counter space and sacrifice movement (or risk injury knocking against the counter).

As far as counter surfaces people nowadays don’t usually go for solid surfaces unless you want cheap. Solid surface is not as “hard” and it will sustain scratches. When you are talking about quartz, you are talking about quartz composite? Quartz composite can have quartz content from 65% to 95% (or higher). Lower quality cheap China imports may have lower quartz content and hence less durable. But quartz content is one factor, the other factors are the bonding resin and if anti bacterial materials is incorporated. You are not going to get much info on how good the bonding resin other than it is reflected in the price. Products such as Silestone or Caesarstone are two examples of high quality brands with published specs.

Not many people use natural stones such as marbles, quartz, etc.  as counter surface. They are natural stones and hence more beautiful but maintenance is a bitch. But that’s an option.

As far as floor tiles are concerned, technical porcelain is the type you should use for kitchen floor. Ceramic tiles are just not good enough for high traffic/high use area such as kitchen. Wall tiles you can choose any type or design that you like but not floor tiles.

Lol. I can be the spokesperson for Silestone. Both my houses are using Silestone for wet and dry kitchen and laundry area. 😁

Cheap China quartz will have lower quartz content and thus will likely more susceptible to stains and scratches. I have not seen any scratch on my Silestone despite years of usage. High quality countertop you can prepare food directly on the surface (if you desire) due to well sealed surface and anti bacterial properties.

Another option is 2x2 or 2x4 porcelain tiles as counter materials. They are much cheaper if you don’t mind grout lines between tiles. Porcelain tiles are super tough too.

As per technical porcelain tiles. These are the “best” porcelain tiles in terms of temperature, moisture, stain, chemical resistance, impact tolerance. They are more dense and thus are very tough. You should use them in high traffic and or high moisture areas. Of course they are more expensive too!

Not sure if you get the right page, but it is Cubix that I mentioned and not Cubic. Probably two different companies. I use plywood carcass and nyatoh door for my KC.

18" gap is small. 3' is the bare minimum. And I noticed that your hob carcass is huge. What is the size of the hob you're planning to use? The carcass for my 5 burner hob is only 900mm. Maybe you can reduce the size the to increase the access path. And also what is your sink length? Maybe can reduce few inches on that too. Where do you plan to put your washing machine? If the washing machine needs to pass your KC, lifting them to pass the access gap might be a problem especially if it is the heavier front load washing machine.

Go and look at some new porcelain tile designs. Some look like marble with beautiful grains! That’s for countertop. If u need only 48”. They have now 2x4 size which is perfect for you.

For kitchen floors, try to go for easy maintenance gray or dark gray colors. A Niro 2x2 when I bought it last year was about RM21.5 per piece for my backyard of 300 plus sqft. Niro technical porcelain is one of the best. MML is good too but expensive. Guocera, White Horse others depending on the product ranges.

I am using sintered stone for my wet kitchen tabletop, so far so good, very easy to clean and hard to scratch the surface, i even chopped garlic and vegetable on it without chopping board... the con is i dropped a few plates on it and it broke them merciless