They would be dodging the issue if they say it depends on the drill,
we're talking about DC drills here and their speed is set by the power
supply - after all, that's what they are selling, a variable speed supply.
I would not trust the Proxxon to go down to 0V without checking its spec
sheet. It's much easier to make a 3V-18V supply than a 0V-18V one,
there needs to be good current regulation and better components to avoid
burning out your supply. As volts approach zero, current demand from a
simple resistive load goes towards infinity - which without current
regulation means the weakest component burns out first.
I've posted a link to the Walter tool above. Sorry I didn't look for
that one first - but I've used my Minicraft for years with a supply I
lashed up myself. It doesn't go to zero, but goes low enough that I can
cut without melting.
The same supply powers my low-melting-point soldering iron, which gets
used for soldering white metal components together, using the
appropriate solder and flux. It's a little 12V iron, so I can control
its temperature easily, quite important with white metal.
Rapid are a big company with a solid reputation - I've used them before
- so you should be able to return the Minicraft kit if it's in a fit state.