You might try Wheels for all to see if they have any advice.
http://www.wheelsforwellbeing.org.uk/I guess you need to be considering braking and gear changing.
My hunch would be to stick to 1 gear lever, so hub gears or derailleurs with a single chain ring - you can get pretty wide ratio cassettes these days if you need a wide range of gears. ditto with hub gears. Derailleurs can be had cheaply (and expensively!), hub gears tend not to be cheap.
Operating a rear (ie right hand) gear lever with your left hand could be problematic. Twist grip gear levers can more easily be fitted on the wrong side (it just means you twist the lever the "wrong" way to shift up or down).
Rohloff make a twist grip lever for left hand use (normal Rohloff gear lever is RH), listed as article 8207 :-
https://www.rohloff.de/en/products/spee ... index.html.
If you are not familiar with Rohloff, excellent, but be sitting down when you look up the price!
If you want both brakes off 1 lever, then levers operating 2 cables exist (I've only seen these on (mostly old) tandems, the occasional Pashley tricycle and oddly, once on a Kettler Hybrid - with back pedal brake as well - I think this was a German one brought over by the owner).
I guess with the right plumbing, hydraulic brakes could do both wheels off 1 lever. Hydraulic brakes are mostly disc brakes, but Magura do effective hydraulic rim brakes.
If you want something nice and flash, I would try something cheapish first, to modify/tinker with so you know what works for you before splashing out.
Hope this helps.
