I'm an independent freelance senior software engineer. Have been in the industry for a decade and freelancing for most of it. Here's some of my thoughts on the subject...
Seems you're getting lots of opinions here that are sounding cut and dry. But, it's not that simple. Obviously, I'm going to say go with a single person because this is what I do with start-ups. I would also say that because for me, as a person that has done this for many years, it's very straight forward to implement a project like this. Other independent programmers, who knows. It all depends a 100% on who you get.
The best advice I can give you on how to vet tech people is this: don't get swindled by the perception of knowledge by someone that makes you feel silly by asking technical questions about your product or anything else. I've seen many programmers try to make themselves seem smarter by hording information. Make sure you work with someone that can answer all your questions and wants to.
Make sure you pick someone based on integrity as well. You will not know how your code is developed. You wont know how secure your data is. You won't know whether there's a lot of bugs. You won't know a lot of critical information about your project...unless your developer is a person that values integrity.
Vetting a developer technically? I don't even know why you would need that. I mean, if they can't do your project....how will they do your project? You can definitely ask for their portfolio of stuff that they've done in the past, but, you can even get someone who is starting out and build your project/MVP as exercise. It all depends on what you want from your developer. And, getting another technical person to vet your developer? That's just silly. That's double work for you. You have to find two different people now.
I don't believe in giving developers a test. If you do give them a test, make sure it actually applies to what they will be doing. Whenever I get a test that has nothing to do with the work I will be doing, I get really pissed. Any time I need to vet a technical person, I talk about their experiences, ask them open ended questions about the projects they've done. When they advance to a face-to-face interview, I sit down with them and show them a project/code that is similar to what they will be working on and discuss the code and what it does. Someone could be very good at what they do and fail a silly programming test. Plus, it's kinda disrespectful to be tested, for one, and tested by someone who doesn't know the subject, for second. Do you give a test to your accountant?
Building a start-up, you will need access to your developer on and off on a as-needed basis after your MVP is completed. For this reason, an independent developer is much better. Companies have many projects going on. You will need a dedicated person to answer your questions and concerns, fix bugs, build new features requested by your clients, etc.
Definitely go fixed price. Get your wireframes down. You want to do this even if you're going hourly. Why would you ever want to start development without having complete plans down? Later on, after your MVP is completed, you might switch to hourly for new features, etc. However, even here you should get estimates for small pieces of work.
For the love of god, whom ever you work with, before you pay anything make sure you get the code you're paying for. Make sure the project runs on your hosting account as well. I hear a lot of horror stories where the developer was holding the client hostage and demanded more money. They were able to do this because all the code was on the developer's hosting account.
Lot's of scattered thoughts here, but hope they help. Feel free to reach out if you want to have more of a conversation on the subject. I'd love to give you more pointers and answer any further questions you might have.