Sub and base prep for access and drive

[New to this]

Hoping you experienced folk will guide me; how do I research / know what is the 'correct' ground prep for excavating my access (needs widening - a lot - from single track lane onto site) and putting down the hard standing that will eventually be finished as driveway and also the 2m perimeter around the house for scaffolding, and later for pathways?

 

I haven't chosen a final drive surface yet.

 

Two builders have given figures but I can't compare them as one goes down 300mm while the other 400mm (this is to lay geotextile, then hardcore, then Type 1 compacted, in case that makes any difference).  I want to spend the most minimum I can on this; it needs to last once driveway is topped with whatever the final finish is, but I don't want to throw money I don't need at it either.  Ground is very high plasticity clay, if that makes any difference.