Introductions for new BU leader in a challenging situation

Hi,

I’m starting a new leadership role as Head of BU for a team of 40, with 5 direct reports in proffesional services.

Leadership is new to me at such a strategic level.

In addition, an ex-college works there and will be a direct report. We have a bad history, as I beat him to promotion at my last job and he became very toxic and then left. The new company I am going to, he went for this job as well and I beat him to the position again.

He has been disruptive again and has been bad mouthing me and stating he doesn’t believe I am the right person for the job (I am 15 years younger).I start Monday.

So, a very high pressure situation, and it should be a dream job and promotion, but instead I am filled with imposter syndrome and worry. 

I am not sure how I should act to the team, for example should I be serious at all times to demonstrate gravitas and also be positive, or be more laid back and more myself. 

Also, he will be disruptive in meetings, so will undermine me from the offset. The team are all older so will be questioning me as well. So a difficult situation to navigate. 

Any advice on what my approach should be? and how should I present myself?

Also, advice on the below would be really appreciated.

Introductory Email
How long should it be? What topics should I cover etc. I feel as though I need to tell them who I am and about my history, to negate the bad press.

1-2-1 and Group Introductions
I may have to do a mixture of 1-2-1’s and group meetings while in the office on my first day. Same again, how should I approach these, topics and lengths, tone etc?

I have great plans for the BU and want to lead them to sucess, but I'm now thinking if I have made a really bad career move.

All advice is greatly appreciated.
Thank you in advance.