Not a solution for Richie, but may be useful to others:
(Richie, see the edit in my next
post for answers to your questions)
I have a cheaper suggestion than your proposed power supply, that uses pre-built components, and you can re-use them for other purposes, too.
Get a 12 volt battery charger -- that preferably can supply 6 or more amps continuously. It MUST be a transformer type old fashioned battery charger NOT a "computerized" or regulated modern type. You want the type that had a built in meter, not LED lights.
Then get a Harbor Freight Router Speed controller. These are regularly on sale for about 20 bucks or less. That's it besides a bow, and you don't have to wire anything.
Just plug the charger into the router speed controller. Now you can adjust the charger's output with the router speed controller.
The other advantage of this system is that the HF speed controller comes built in with a fuse, cord socket three way switch for full power, variable power and off. These features are often missing from homemade setups, and with the box usually cost more as components than the HF unit does complete.
You can also use these things individually to charge a battery, or add variable speed to a router, as originally intended!
ps that setup would work well with stainless steel wire leader material of about .023" diameter and a 36 inch bow.
If you want a different material, diameter, and or bow length, you can use my hot wire calculator to figure out what will work. If you can't use the calculator, I'll be glad to help figure out what you need on that thread. As mentioned earlier by CayminLast it's at: