Heavier gauge spokes don't always make a wheel stronger. They sometimes make it weaker. When spokes stretch (very slightly), they relieve the spokes that are bearing the most tension. It spreads the tension around the wheel. This has been proven. You want the lightest gauge spokes you can use. 14/15/14 might be ideal for you. This is one application where butted (aka swaged) spokes might make a real difference for you. They cost more, but they could prevent or delay the need for a new wheel.

I'm glad to hear about your shop, but I'm also glad you came here for advice. The shop may not suggest (or even agree with) 14/15/14 spokes. It's what I recommend.

I agree with Aaron in recommending the CR-18 rim. It's not too light but not too heavy. It can withstand a good amount of tension, which you want.