First step to restoring my 1974 Duster; Front End rebuild (coilovers needed?)

Coilovers are nice but NOT necessary. The stock torsion bar suspension works very well once you learn how it all works. It's biggest drawback would be that the bars get in the way of many types of headers... but companies have been making headers that fit for many decades, there are still plenty of options.
My point is, if you really want to spend the $$ on the coilover conversion that's fine, but don't let anyone tell you that the stock stuff is trash.
As far as disk brakes go, I have stock style disk brakes on the front of my dart from a '73 duster and I feel like they work amazing for what I need them for. I do have the larger 11.75" rotors but even I will admit that they aren't necessary for my style of street driving.

That link that you posted would be basically the route I took and the only thing left for me to do is add the sway bar. I have been happy with my dart so far but the next big upgrade would be to swap out my steering box with a borgeson unit (I hear very good things about firm feel as well).