1980's bike vs New(er) Models? (Should one stay with an ancient bike?)
I suggest that you go to your local bike shop and see if you can test ride a modern bike.

As Daniel says $500 will not get you far. A test ride of a couple of bikes at different price points will help you decide. You may find that your "retro" bike is OK after all. Or maybe that it never did fit you properly. But a new bike could be 5kg lighter than what you have!

Now is a good time to do it, because it's Spring there and the shops will have a good range. If you decide to buy, then you can save for the next six months :-)