Hi Tabby!
First off, you are not alone. You would be surprised at the number of overweight cyclists. It's called 'the fitness paradox". Bicycling is a very forgiving endeavor, it welcomes just about everyone regardless of age, weight, bum knees, or fitness level.
If you think the bike shop is condescending is there any other shop?
I have no idea what the price range is but wouldn't steel be a good option?
What do you think, Lugged Steel Ladies?
2008 Trek FX 7.2/Terry Cite X
2009 Jamis Aurora/Brooks B-68
2010 Trek FX 7.6 WSD/stock bontrager