You may also have to redish the wheels if the OLD is 126mm and you cold set to 130 for the modern hub.
FWIW, for my own upgrade/conversion I kept the classic quill stem and handlebars. You'll have to get an adapter if you go with the threadless type, and then new handlebars and the stem itself, further increasing the cost.
Your upgrade should be based on your budget, and whether you're going to go used or buy new parts. If you go new with a 4600 groupset+wheels+cockpit components+pedals+seat+post, you're looking at a minimum $650-700. Is this worth it? Buying used will be cheaper, obviously, but a bit more work on your part.