The napkin math is discussed in Millionaire Real Estate Investor.

Heck you can write up a mobile home purchase agreement on a cocktail napkin:)-As already mentioned-mobile homes come with a title much like an automobile title. 

Just make it safe sound and sanitary.

Adding up the figures on a napkin suggests that $15,500 would be eaten up by ongoing expenses, and $11,500/y for a 20 year 5% portfolio loan.

Depends on the city and Government regulations, and how many lots, you may have to invest in common utilities/facilities (roads, parking, water, sanitary sewer, fire water requirements, storm sewer, to mention few), making the cost per sqft higher, and that's a chunk of $$. 

10% is a good back-of-the-napkin figure, but depending on the state of the property, you could be looking at a lot more or a lot less.

I have heard of purchase agreements written on napkins holding up in local court here so not sure if my fear is warranted.Does anyone know of any specific requirements in California that I need to worry about, or is the contract I have good enough?

Some rough back of the napkin numbers:Find a SFH, 4br+ for $380k or less.

After a receiver has completed the work on a property and fixed all the sanitary code violations, do they get title to it?

Get so good at running numbers that you can work it out in your head or on a napkin, so to say.