Pre-made assets off the marketplace definitely won’t cut it. Not only will people recognise the assets since they appear in other games as well, but there will be no consistency in your art direction, in terms of style and quality of the assets.
You’d end up with the captain of the ship looking like a space marine, the first officer looking like a near-future police officer, and the crew looking like low-poly characters from The Sims.
Technically it is possible to create it all with a small team, but you would run into the problem that development would take years (think 10+ years), and unless you’re paying your team, it’s unlikely anyone would stick around that long. Unpaid projects tend to have a very high turnover of team members, so expect to be replacing team members every few months, further slowing progress.
As I said, it can and has been done before, but expect to scale down your expectations (and the size of your game). I would also anticipate doing a lot of the work yourself as you won’t be able to rely on others to do it a lot of the time.
Good luck!