If you really want to ride like a madman a front brake is a good idea,especially when you consider thefront/rear brake ratio of something like 60/40 or 70/30.
Even the best rear brake set-up only can lock-up the rear tire.Although I agree,a spool wheel looks best.
If you want to upgrade your bike with a disc brake set-up nonetheless,I'd suggest to look out for XS 360 and XS 400 wheels.The early versions even had wire wheels and the installation is pretty easy as most of the parts are the same within the XS models.Those wheels I'm talking about are 18",which I like but it's also no problem to lace a 16" rim and this stuff is available pretty cheap.