Old bikes ride nice and look great. However when you start fixing them up you'll have to decide on overpriced used stuff from eBay to keep it all original or put new modern components on it... if they'll fit.

Either way, you'll end up putting more money into the bike than it's worth. Maybe even more than what a new bike with a decent modern group set.

However if you are looking at it just for the educational aspect of learning to DIY your own bike, then maybe well worth the expense. Having done it myself, I wouldn't do it again.