I've done about a dozen of these conversions for friends and many with single gear up front as well. Mostly I use 80's Japanese bikes with downtube shifters as it simplifies everything but I did a 3x9 once and used mtb Deore XT shifters. The thing to watch for is cable pull. Shimano 11spd rear I don't think are the same between mtb and road so you can't just use, say Deore M8000 shifters. On the front, derailleur pull is also most likely different between road and mtb. Although you can use a single gear on the front, I usually used the smaller gear (39 or so teeth) and mounted it where the large gear was then use a shorted bottom bracket to bring it closer to the frame and grind down the chainring nuts and screws. Easy with old bikes as I have a box of parts but if you need to experiment with new parts to get the chainline near center of the cassette it may get expensive. For brakes, all modern road bikes except the new Shimano SLR use the same pull ratio as older cantilever brakes. I usually use Shimano BL-R550 levers as they are great quality, come with all cables and in black or silver. However if you have slr brake levers I'm not sure what flat bar levers are compatible. Although I think this is a great conversion for city use, make sure you know what you are getting yourself into as all these conversion factors can quickly make this a nightmare but perhaps others can chime in on compatible shifters and levers. Here's a pic of my last conversion, a $70 Nishiki to 9sp x 1.