Up to you, I would just avoid big instances like .world or .ml. People do congregate on big instances in most of the fediverse, so IDK that "professional" enters into it. It's not as if you're running a law firm on a @hotmail email address. I like hosting stuff for myself, so I am running my own instance.
For yourself you could get away with spending around $5-$10/mo, plus ~$10/yr for the domain name. More users/load would need more resources, .world is spending >$150/mo for the server(s) alone, and that will only grow as the instance grows.
Big thing would be site-wide moderation and managing federation. Dealing with reports, illegal content, communities that break server rules, users that are harassing others, etc. If you slack too much on that (or have overly lax policies) you may end up defederated by instances. Making the decision to defederate other instances. Etc.
Entirely gone.
Mostly just changes what you'd see on local. Federation can be wonky/slow at times, but that is true of federation between big instances as well, it's just something you have to get used to when using Lemmy.