@Nick Harz Great questions man! The short answer is the fact that you are asking all these questions means you don't know what you are doing, and you absolutely need to partner! However I'll do my best to answer one by one.
1) Can R3C zoning be subdivided?
Don't know this, you will probably be able to find this by digging into your county/city planning and development website page. I definitely think it is worth a call and email to that department as well.
2) I read that if I subdivide a land to more than 4, then it has to be a subdivision. What are the cons for that?
A subdivision has to have an HOA / Condo association depending on what it is. There has to be additional planning, document filing and just more paperwork. This is totally worth it if the money is there by adding additional homes or structures. Just be ready to have a real estate attorney and insurance company to help!
3) Should I partner with a developer or builder?
Absolutely it sounds like you haven't done a bunch of these before which means you will need help. I would either partner or sell this / flip it to someone who does know what to do. From the sound of it someone is already selling it so there probably isn't a spread.
4) What due diligence can I do before buying the land?
Check the city zoning laws, check the future land use, is it in a historic neighborhood? Will a neighborhood association or councilmen even allow you to do this? Do you even have the money/cash to fund this development? How much would each dwelling/structure/home sell for? What will the floor plans / finish out selections be? Has anyone done this nearby? What was their experience?
5) Which home builders are appropriate for a project like this?
Any home builder who has done it before and is willing to partner or even take on the project. Experience and quality aren't cheap though so be ready for that! Make sure you are comfortable with their process and how they work, it will be a long relationship with many obstacles so make sure you are aligned.
6) What else to consider?
I don't know the area and the projects around there but you really need to consult with people who actively do this and get their opinion.