While I'd not hesitate to say that Singular and On-One bikes are lovely (they are, I've had an On-one and I covet a Singular 😀 ) you might want to consider the Surly Karate Monkey. It's heavier than both the On-one and the Singular, but it'll build into anything you could reasonably want from a frame, flat bars, riser bars, drop bars, gears, no gears, 23c tyres, 2.5in tyres with mudguards. It'll do it 🙂
I'm just off to South Wales with mine for a bit of trail-centre fun (SS MTB build on mine) but I know that you can Alfine them quite easily because I've just Alfined a Surly 1×1 and the dropouts are the same. They sometimes come up S/H like mine and you can pay £150ish instead of £400+ for a new one. I like mine, it's a bit "sturdy" but it just rides like a bike should, it's fun.