Need a little more information about the marketplace and which team you're referring to, but my short answer would be that you would need someone providing the following components:
2) Financial Oversight
3) Product Development/Management/Technology
4) Sales & Marketing
5) Customer Success/Support
These do not need to all be different people, but those areas are the primary areas that need to be planned and supported in the very beginning.
I agree with what many have stated that you’ll need someone ( if not yourself ) that can work directly with the technical staff, to work through process flows and manage features through a SCRUM process ( prioritizing what’s important ).
Without knowing more about your niche, I would make the following suggestions:
If you’re currently not a subject matter expert ( SME ) in your chosen niche, get this person on your team! This is a two-fold approach, the SME can assist with your product development ( what features are must haves etc. ), and help create relevant content in your niche, you’ll need high quality content for any SEO campaign you take undertake. If you’re really lucky a SME can help with customer relations, and customer service too.
Identify how your going to attract customers, is that through cold calling, google ads, social media ads ( Not sure if social media is relevant without knowing more about your niche ), direct mail, youtube videos / channels, trade shows ( if we ever have those again ) etc. I would add someone to my team that has a proven track record in the marketing, towards my target niche. In startups this often describes Biz/Dev role, I am putting extra emphasis on them knowing how to market toward your target customers.
One word of caution on outsourcing your development, make sure you OWN any custom developed code! Make sure your team has access to a git repository or wherever the source code for your project is stored. Some day you may decide to bring your project in-house or transfer it over to another development group, that’s not the time to discover you don’t own the source code ( it happens )!
I believe that after deciding on direction, the idea should be tested first, and based on this initial mini-test and the speed at which you wish to move, you would have to compromise and find the middle solution to remain lean. Keeping it reasonably low cost, lean and focus & goal oriented is essential for success not only in startups, but also in any fast growing company.
Since you as founder are in charge of leadership and management (i suppose), the two main things I would bet on are development and sales. Everything else will some naturally. In addition - find a couple of good consultants and make a deal with them - they not only add experience and tested knowledge, but also bring in their personal networks which can be truly valuable. It's also a positive sign for investors (to have some reputable names with proven track record onboard).