Questions about Spaceships

[Nyarli]

First of all let's assume, that I ask about hard-sf(at least in regard to spaceships) setting, like Transhuman Space.

1) High- and low-thrust engines:
If I want one engine to work as both high-thrust and low-thrust, then realistically it should be available
a) only at TL 11 as reconfigurable system
b)at TL of introduction at no extra cost(this is just settings of engine, not design)
c) Something in between?

2) Same question about different reaction mass( hydrogen, water or whatever).

3)Ramscoop:
a) Is it reasonable to allow multiple ramscoop systems(to represent one big, like space sails)?
b) Can it be used with high-thrust engines?
c) Can it be used (again - realistically) to refuel( while moving on sufficent speed with engines shut off)?

4) Frontal DR
Even assuming zero micrometeorites, there should be some amount of required protection to move on large fractions of speed of light (and such speeds are achievable with ramescoops) through interstellar medium.
a) Does anyone know some approximations of such requirements?
b) If spaceship use ramscoop should such requirements be lowered, waived, raised or unchanged?

Thanks in advance.