36t should be pretty common and they can be swapped out. I'm a fan of race face single speed cranks which work well for 1x setups. pretty much every single speeder on my team, 3 of them, like race face for the value of them. They're good and affordable.
I suspect the reason you LBS is saying to change out the BB is because it may likely be an octalink BB which makes finding a crankset ore difficult. External BB's are more common. Doesn't mean there's not a crank to fit, just fewer options. Pretty much every brand supports external BB's at this point. External BB's will fit standard BB shells, there will be directions on where to place spacers when installing the BB, if you don't have the LBS do it.
Single ring options: http://www.raceface.com/components/ring ... ngle-ring/
Single Speed crankset: http://www.jensonusa.com/!6bJO!IVL0-hCl ... 4Aod0zoAcA
Plenty of other options available from them and other companies, the evolve is what I have and it's been solid. I bought it from a "retired" teammate and he raced it last year and loved it. The next and turbine are lighter but are 2x, you may have to convert to single and bash. The next SL and sixc are in the stupid light category of cranksets, the sixc is pricey but light and according to the website, intended for all-mountain riding suggesting it should be pretty solid despite the weight. Sixc also has the option to come stock with just the 36t ring.