Grace, it will all depend on your state and the local practice.
Your first step will be to find an attorney. Many will do an initial consult for free. Find one that specializes in family law and not one that does family law and a million other things. The attorney can help you figure out which type of mediation might work for you.
Parties do sometimes attend mediation without their counsel, but you should know what you are entitled to going in by speaking with an attorney and not sign off without attorney approval of the deal. Also if that is the case it should be understood that neither side will have counsel present. Given your situation, because the children are adults, you might be able to just hire a financial mediator. My counsel recommended this before she understood what mine was up to.
Keep in mind that while everything seems nice now, this is probably his way of getting out of the door as nicely as possible. Mine promised that the kids and I could stay in the house and he wanted an amicable divorce. Two years later and all bets are off. Best to get the finances tied down first. If you can, do an actual separation agreement. Even if the statute doesn't provide for them, they may be enforceable as a contract. That way you get the best of both worlds, a resolution while he is in a more generous mindset and if comes back, you need never do anything with it except keep it in a drawer in case he does it again.
Check into your options now. Don't wait until later. Information never hurt anyone. Talk to several lawyers and find one who is a good fit.
