I wouldn't compare building a 1911 is like making the molds to make legos, but it would be like if you had to hand fit every lego to fit into the other ones.
If you can get a nighthawk for $1300-1600, then you shouldn't be building guns.
I had so much fun building my first 2011, I started building my second one before finishing the first. They're addictive. I'd like to do more.
As to building on a RIA gun - it's a RIA and will always be a RIA. That's not a problem for a shooter, but it is a problem if you want to charge for your upgrades at some point down the road when selling it. Is someone going to pay 1K+ for your custom RIA? It's possible, but I'm not sure how easy it would be to find that buyer. Norinco's have a great rep for solid frames. Maybe RIA will attain that level of appreciation someday, maybe not. Right now it's a gun that sells for $350 when the cheapest Springfield is selling for 550. Lots of people build higher end guns on the bottom tier springer frames.
Mill is needed for doing slide serrations or other slide mods, mag well chamfering the easy way, making the butterfly cut or Nowlin or Para cuts, raising the trigger guard (I forget what that cut is called - I'm currently on drugs

), golfballing, and barrel hard fitting. Lathe is needed for fitting the barrel bushing. Either way, you'll want a rotary tool of some form, good sharp fine cut files, and a lot of sandpaper.