vintage V72, is it viable to re-furbish it? issues?

[stenerlov]

That screwdriver hole you are talking about on the front - that is not gain.
That is the balance adjustment for the unit - to get the lowest hum-level.

[melonmachine]

there's lots of these units around and working so not at all out of the question. BUT......
The valves or the transformers could need replacing. You may have some other minor work to do. That means 300 euros is way too much.

You'd probably want to at least do a gain mod too which puts the cost up more.

Look for one thats working, they're not that expensive, although have gone up in recent years