Advice on hand-me-down suit. Can it be not a costume?

If it has been in the family for a while, it's worth keeping for sentimental reasons. Whether you wear it is up to you. With lapels like that, you would need an extra wide tie to balance it out. You can have it altered if you have a good tailor, but if you want a contemporary look you're better off buying a new suit. I would think part of its value is the authentic look from the time period. Best of luck.

Welcome to the forum, first off! This suit is obviously from around 1975. I wouldn't advise wearing this to a working environment or any kind of formal occassion, for the simple fact that the details, fabric and cut are all from that era and look out of place in current times. That being said, I have plenty of musician friends (mostly traveling busking musicians) who wear these suits with confidence and have made it part of their "look". In regards to what people have been saying about slimming lapels and pant legs, etc.; I think you would truly be wasting your money. No matter what you do to this suit it will always look like the era it came from because there's no changing the fabric. If this has significant personal meaning, then sure; keep it around, if not, I would advise buying a modern cut. You have to also understand that even when fashions come back, they come back anew with new innovations in fabrics and details that give it the modern aesthetic.