Oxygen concentrators dont have to be modified at all. Pipe them to the torch and run the hell out of them. I run a 5 liter machine at 4 liters for better purity.
They are made to run day and night and keep people alive so a few hours feeding a torch is nothing. I have 2 that have 64,000 and 54,000 hours and they are going strong.
All the brands you named are commonly used so get the most LPM you can. None of them are lemons.
I run a GTT Phantom on 3 oxycons, its a realy nice torch. All of 2015 I spend $30 on oxygen.
https://www.youtube.com/watch?v=JmfymeMiJyk