What to do first?

Front disk conversion, absolutely. Rear disks, don't waste your money, they are not necessary unless you just want bragging rights.
And get your front suspension rebuilt, with whatever system you decide to go with. Then add the new motor and tranny.

Most important, do these things one at a time so you can keep the car on the road while you make improvements. Lots of guys end up tearing the whole car apart, and then get overwhelmed or run out of money and never end up finishing the project.

Just my two cents.