I successfully rode stock single wall rims on my first mountain bike (07 Raliegh Mojave 2.0) while I was 245lbs. The front rim lasted 300 miles before it tacoed (totally my fault) and the rear lasted until I sold the bike at 1200 miles. In my opionion, they are fine so long as you keep them reasonably true and have them retentioned after a couple hundred miles.
I think your bike shop is tring to upsell you into a more expensive bicycle. If you are looking at entry level bikes I humbly suggest you buy as much bike as you can upfront. Upgrading as time goes on is expensive and a lesson in dimishing returns.