I don't have any experience with boost gauges per se but I've certainly spent some time monitoring the boost levels via INPA etc.
INPA indicates absolute pressure (in hPa), so it never goes negative (a vacuum) - but some mechanical boost gauges may read pressure relative to atmospheric and so may read a vacuum (negative pressure) if the absolute manifold pressure is below atmospheric pressure.
Which gauge you get depends on what you want to know - absolute pressure or relative pressure.
And I'm not sure if there is a boost hose somewhere you can simply snip and "tee" into - I'm pretty sure the MAP (manifold absolute pressure) that the DDE uses is bolted directly to the inlet manifold.
FWIW, I think the boost pressure max is a little under 2 bar (26-28psi).
