Regarding 2wd and freewheel:
When you have two wheels side by side, the outside wheel will travel a longer distance than the inner wheel when turning.
If you stick both wheels on a solid axle, one or both wheels will have to slide(or "scrub") a little everytime you turn.
Apart from the unavoidable wear, this makes steering heavier. Solid axles are rarely used on trikes, as it's not immediately compatible with regular bike parts.
Easiest way around this, if you want the paired wheels to drive, is to have only one driving and the other freewheeling.
This gets rid of the scrub, but steering will be different depending on direction of turn. With the drive wheel taking the outer(longer) path, it'll help steering. With the drive wheel taking the inner(shorter path), drive will fight the steering. This is often seen as good enough for trikes intended for sedate riding.
The elegant way around this is to create some sort of differential. This is a mechanical device which allows a dynamic distribution of drive between the paired wheels. Balanced steering and no scrub, but mechanically more complicated, heavier, and more expensive.
While there's nothing wrong with caliper brakes, discs have the potential to be far superior. Brake performance remains consistent in all weather, no wear on rims, and spare parts supply isn't much of an issue.

Custom work will always be expensive, no matter what. There may not be much point in asking the builder to incorporate the comfort bike into the design.