You're a tad on the heavy side, but you're only just an outlier if at all. The fact is, your bike shop is just sh*t.

Selling a bike to a big guy should involve bringing at least the rear wheel up to tension, unless of course the intention is to keep shafting you for $20 a month.

IMO almost any cheap 32 spoke wheel should survive under you, as long as it has enough tension. Spokes break because they get slack under load, the most important component of any wheel is the build, and inexpensive (not ultra-cheap) gear is often perfectly durable when given a chance.

If you can find a co-op nearby I recommend using it to escape the cycle of dependance those bike shop turkeys have you on.