The blueing will be better than a new gun, and a proper tuning kit will make it perform much better than a new gun as well.
Some people say that Weihrauch quality is not as good as in the 'old days', and quality was excellent in the 1980s, so you probably have a good one.
All you are really missing out on is a polystyrene box and a warrenty, which basically means that you are missing out on cluttering up the environment with non-degradable plastic and having your rifle deisel for a year because you can't strip it down and relube for fear of invalidating your guarentee. My HW77K is from the late 80s and I put a Venom Stage 1 in it and I wouldn't change her for a new one ever. Do read Spring gun tuning article in the Advice and Tips section.
http://www.airgunbbs.com/forums/showthread.php?t=115108
Go ahead and tune her up!