Focus on performance marketers who have verifiable performance. You want someone who speaks in numbers (e.g., ask vlookup vs index/match type questions) and is driven to test-- so look for someone who can explain different strategies (e.g. alpha/beta, how they segregate kws/match types, thoughts on long tail, negative management, manually managing bids vs automated systems, campaign structures, even basic things like constrained budget allocation) in a manner you feel comfortable with. Given your niches and scale, maybe place more emphasis on testing novel channels to help establish both CACs and LTVs, because much of automotive search will be likely weirdly high funnel because it is a highly considered purchase category.

Marketers are going to be good self-promoters (it is their job, after all), and the good ones are worth a lot because they can save multiples of their comp in wasted spend, but most are unfortunately flunkies who parrot Google's talking points, always use automated bidding and constantly change KPIs.