How big of an instance? Hoe many users do you imagine or intend on hosting? What kind of communities? Do you have a specific theme or raison d'etre?
What sort of tools or support do you think you will need? What kind of tools do you think your users will need? How familiar are you with other forum-like platforms on the fediverse?
I would make a list of all of your needs and then do a comparison of Lemmy, PieFed, mbin, nodeBB, and other group-focused federated offerings to see what makes the most sense for your imagined use case. They all have different strengths, weaknesses, and requirements. Set them up and test them on different subdomains so you don't pollute your namespace on remote instances while testing.