Welcome, as you discovered you will get a lot of opinions all right and all wrong. It is mostly a matter of preference as there is no one way to do this.
I might point out that I do know this is an expensive hobby and that I am pretty well off financially. It is merely my thrifty personality that likes to save money where possible,
I personally would buy mostly dry rock that looks nice and add some live rock. In a couple of months you will not tell which is which and in a year it will make no difference at all. All of my rock was dead because I collected it myself but had to bleach it to bring it on the plane. (It was legal then) The bacteria is what you need on the rock and it will grow no matter what you do.
As for the size of the tank, that is up to you, mine is 6' long and only 14" high, it works fine.
Some people like sand, some gravel and some nothing, it all works and is what you like looking at. There is no need to clean the substrait, you may want to suck out dertitus if it forms and you will see that much more with sand although all healthy tanks make the stuff. Most of it comes from dead bacteria and rock boring organisms. You will be amazed at the amount of detritus those little guys can dig out of rock. The stuff is not bad and won't hurt anything as it is mostly inert.
You will read that it contributes to nitrate and maybe global warming but I don't think it does anything, if it did my tank would have crashed decades ago. But you may want to eliminate it for looks.
Read everything, then use common sense and look at the age and looks of the tanks of the people giving you advice.
There is a lot of talent on here but an awful lot of information, not all of it 100% correct as it is all opinions and there are no experts because it is a hobby.
Have a great time on here.
Take care.
Paul