The IRS normally requires 6 years of back returns per the IRM. I would only do more if the IRS asks for it. Each state differs so you may need to check on that. Maine just follows the IRS rule of 6 years.
As for voluntary disclosure, Eric had a presentation on that not long ago. I think he said that the IRS no longer wants that if the client did not engage in bad behavior (tax evasion or income from illegal sources I think). I don’t remember if I heard this on a webinar or podcast. You might want to dig a little regarding voluntary disclosures.
Good luck, non-filers can be a load of work. Get paid first!