Local has come a long way, but it is still limited and slow. And while there are some people who have done stuff like this, the field is so new that you're probably going to get someone that doesn't have direct experience with everything. In other words, they're going to get stuff wrong. You will have to rebuild some part of it. You might not purchase the right hardware. Can you live with this?
In all fairness, though, if you have someone who has experience in evaluating new systems and using them to build something, then you can still be in good shape. I mentioned this, simply because it's a skill that is not as common as we would like in this world. Just look for someone with a track record of delivering functional software using new technologies.
My personal bias is that I love to keep as much local as possible, but I also realize that I bought a $3,000 machine that so far has saved me $5 in tokens from an external API. As I see it, the only real reasons to have local AI at the moment is privacy, but that does fit your use case.
As for a turnkey solution, they have their benefits, but their moat is significantly smaller now than it used to be. Quite frankly, you can vibe code the majority of TurnKey solutions in a weekend. Well, at least the parts that you need.
Sorry to not give more specific answers, but a lot of your questions may depend on whichever developer you decide to use. There's not necessarily a wrong answer in many cases, there are multiple paths to achieve what you are trying to do. If I were you, I would focus on long-term maintainability and security of your system. For example, you can have the best thing in the world, but if you can't pass a SOC2 (or, even worse, your developer has never heard of something like that) then you are going to be in a lot of pain.