For replacing the Weinmann brakes, it'll be easiest if you make sure any replacements are nutted as opposed to the modern recessed allen nut fittings. I'm planning getting a set of Tektro R359 dual pivot calipers for a similar upgrade to an old road bike of mine. These are available in either nutted or recessed fittings and a pair can be had on
eBay for about £30.
Your rear dropout spacing is probably 126mm. Modern road cassette hubs are 130mm but being a steel frame, it can be cold set to permanently respace it or you can just spring it to the required width whenever you refit the wheel. Details by someone who knew a lot more about it than I ever will are here:
http://sheldonbrown.com/frame-spacing.html
If your shifters are friction shifters, you don't need to stick to 6 speed and (I think) a 7 or 8 speed cassette would be fine with the rest of your drivetrain.