To begin with, let me say that "hire a designer" is not really an answer. Problem is, I will still have to evaluate and accept or reject whatever he or she creates. Personally I would go with a totally minimalistic design, but I bet many would consider it amateurish. I don't want to risk losing customers just because my personal taste is not very mainstream.
Where to get an idea what "modern", "professional" and "trustable" mean these days?
Web Website Design Web Design Web Dev
Spend a couple hours reading Smashing Magazine - They consistently post on many things regarding web design and development.
take a look at themeforest.net for an idea. We got our templates and images from them and their sister sites.
http://designshack.net/ has good collection too.
Will also look at themes offered for sale on volusion, shopify, 3dcart etc.