First you need to decide on an obvious "must" before it is wise (financially) to proceed.
Are you truly limited to 4'x8' layout (no wider and no longer)?
An extra foot of width could avail space for wider diameter curves. Thereby allowing the running of longer locos and rolling stock (though, not much)
An extra foot or two of length would make a passing track more practical.
A suggestion on turnouts. Use as wide a turnout as is practical, at least, on spurs. Why go with O36 switch, if an O48 switch would still work. Why go with an O48 switch if an O60 would still work. And so on...