Hi Iain,

Unfortunately, you're in a bad situation. If your Magento store is as bad as you say, no decent Magento developer will take it over but it sounds like your requirements are in-need of custom development and not something like Shopify.

It's probably a hard pill to swallow but at some point you'll realise, the best option is to invest hard in finding a development agency you can trust and go for a full rebuild.