Hello and Welcome to MTF!
3 by 8 is a good size for N. A lot of track. Two transformers give you two controllers if you are coventional and more terminals for accessories.Bigger may not be better because two give you more flexibilty. You could get double controllers and run four trains. This is without understanding what you are trying to accomplish.
I don't know about adapters. If you have wires you cut and solder.
Kato is a package deal because you are buying the kits to make a certain layout. You see this in the smaller scales. It is a convienent way to go. Transformers are the option because of the possibilities . Dcc or traditional.
Bob
Beware some MRC equipment is for the G scale. You need to be careful on what you buy. Some have variable AC, Some DC
