The problem is a power rack used without assistance doesn't have the
'feel' of a manual rack, due to the extra friction of seals, etc.

I'm also not sure how long it would survive without pressure lubrication
as provided by the pump.

Why not find a non power unit to use?  I'm pretty certain some models of
the E30 came without power steering - although of course they may not have
been available in your country.

One problem is that of retaining some sort of lubrication. Another is that a
power steering spool valve chest contains a torque rod connecting the column
to the rack pinion. This means that the valve body needs removing altogether
or modifying to give a solid connection from the column to the rack pinion.

I don't think it's as simple as it might appear, bearing in mind that the
conversion needs to be sound from an engineering and safety POV.

To do the job properly IMO, machining would be required, if only to remove
the piston, so unless you have machining facilities, or are prepared pay for
it to be done properly, I'd advise you look for a suitable manual rack.
At least then you'd have no worries about the racks integrity.
Mike.