I have had a similar experience recently, and I would suggest a few areas to consider which are seamless navigation, quick checkout, secure payment, and tidy product searching. Focus on the basics first (cart, payments, whether you let people track their purchases), and you can tackle the extras like user personalization or push notifications later.
As far as developers are concerned, there are a bunch of options - one I have seen is PixelCrayons which does mobile eCommerce apps, so a fair bet is to get a hold of the case studies and ask around with past clients you inquire with, to see who really understands your business before you make a full commitment.
