If you're going to be using friction shifters, get whatever rear mech you can find that works with the cassette you want to use. Then get a narrow wide chainring that fits whatever crankset you want to use. Done.
For instance: get a new tiagra long cage mech (cheap), find a 9/10/11 speed cassette (check that it fits your hub if 11-speed). If you get the right mech you give yourself an upgrade path too, just by adding the matching shifters later on.
Getting a 'real' 1x SRAM setup will likely be more expensive, SRAM shifters don't depreciate in value much I have found.