Unexpected advice about headsets

Do it yourself. You won't save much money, but you will learn something, which is more important.

DIY press or hammer-and-block-of-wood methods are both fine. I literally don't know what the mechanic is talking about when he says 'seal it properly'... perhaps he meant 'seat' it properly, which is not hard to do.

Nice frame btw, I had a similar one.

Quite!
Bike shop Mechanics are 'not' qualified mechanical engineers (tho' I do know one who is) - as they don't need to be.
Thus, wack it in with mallet and a wood block; it should be a mild interference fit, if correctly sized, and you'll hear when it's seated.