If he likes a deal and wants to get into it, he would need to be able to fill out the subscription documents, which are ~10 pages of things like, name, address, SSN, whether you are using a Trust, LLC or other entity, and a bit of information about his financial status as required by SEC rules. 

I'm also based in California.

If you are willing to put down a larger down payment it might make sense with the flexible underwriting they offer, especially if you are going to run it as a MTR. 

We welcome any suggestions or insights you may have Additionally, if anyone has information on the approximate cost of building a duplex and triplex in California, we would greatly appreciate your input.To provide context, neighboring properties in our area offer insights into the local housing market.

You'll also receive a lower ranking on Airbnb and other listing sites because you don't offer an amenity that everyone else in the area has. 

Considering seller has no tax returns to show he may struggle to sell based off what he claims.

I already feel like I have a good base of knowledge but I'm still a little confused on the legal side of the business and there is definitely more that I need to learn.

Assuming credit is healthy there are lenders that will offer more favorable terms. 

I'd encourage separate units not shared based on my experience as a general contractor and developer at all levels in two different cities not a rooming house 

It’s a great way to build some  base knowledge on any topic you are interested in.Finally,  I would e courage you to find your local real estate investment group.