Magento does take upkeep and maintenance by a knowledgeable developer to keep it in a good state. From what you say, it sounds like there's been a build-up of technical debt (https://en.wikipedia.org/wiki/Technical_debt) which might be hard to "clean up" because there'll be lots of small mistakes in various places.
In terms of hunting for a new developer, I can recommend https://commercehero.io/.
You could look to request an audit from a senior, high rated developer on there. Even if they're a bit more expensive, you might not want to get them to do your regular dev, it would be worth working with them to do an audit so you can the low down on the current state of the site. You can then use that list when talking to any other developer.
It would be good when talking to a developer to have a clear list of the open issues with clear descriptions and steps to reproduce where applicable and organise these into a priority order. This can be helpful for new developers coming on board understanding the scope of the project.
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