I am thinking about purchasing land for the intention of using it as a RV & Boat Self Storage.

I'm considering purchasing a condo or a small SFH in the Oceanside/Carlsbad area as an investment property and renting it out on a short term rental site like AirBNB. 

For the BRRRR process I plan on using a hard money lender for the rehab process for the SFH's but what I'm not sure of is, should I purchase a duplex and rent one unit out and live in the other or should I just rent for now, not purchase a duplex and just purchase a SFH with cash and start BRRRR'n immediately?

Congratulations on your first investment purchase! 

Although the study may increase the gain, the gain may be taxed at a much lower rate making it a beneficial investment.If a property is purchased with the intention to flip or own for a short period of time (less than 3 years), a cost segregation study may not be significant.Overall, as long as you intend to hold the investment property for greater than a year, the benefits of a cost segregation study should be considered.

As Bill mentioned, the best way to decide would be to get a free cost/benefit analysis to help you determine if you would benefit.

Any amount you purchase less than you sell, or any amount of cash you take out is considered to be profit. 

I have purchased an off-market property from an investor who now wants to sell the rest of his portfolio.

The last time I represented a mobile home park purchaser, county health department allowed four MHs per septic tank. 

This helps you determine what offer makes sense to you.