Planning for Potential Costs/Insurance

Hey all. Our Preschooler (3.5) has yet to be officially diagnosed with anything, but his teachers have recommended having someone observe our son in the classroom due to his distractibility/big feelings/etc. While some of his behavior is certainty age appropriate we've always believed it's a bit more than that, so I'm assuming we are eventually heading towards an ADHD diagnosis or something similar. Given that expectation, and now that open enrollment has begun for our Colorado Health Insurance Marketplace, I'd like to better understand what kinds of costs we'd be looking at to best help our son and set him up for success; what kinds of doctors/therapists/etc. we're likely to need, if any kind of medication would be started this young, etc. What kinds of things should I make sure are covered by insurance?

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I'm sure there's a big variance but if I can get a ballpark understanding that'd be helpful.