You may have a grasp on this aspect, but there are items to consider that can play a big factor when choosing your land such as topography, existing zoning & platted lot status, minimum/maximum requirements such as parking, detention, size of building, setbacks.  

Generally the fees are going to be very rough on DSCR Loans of that size (as a percentage of loan amount) but probably no choice right now - the market for DSCR Loans under $100k is very small and less than $75k even smaller. 

LP in other hand, has bigger size of downside with limited upside.

What is your experience / track record with this type and size of investment? 

We are sandwiched between well known hot spots and its an ideal location for someone who wants to explore the beach life or any of the down-town areas nearby (Los Angeles, Long Beach, Redondo, Manhattan, Southbay Etc), new build with modern amenities, washer/dryer in unit, full sized fridge, dishwasher, professional range cooktop, queen bedroom, full standing shower, large kitchen, full sized couch in the living room, private front patio. 

I like the pontoon boat rental idea as an add-on service for your STR - on decent-sized lakes, pontoon boat rentals can easily bring in $200-250 for a half day rental or $400-500 for a full day rental, so you could make a tidy little profit off of this kind of add-on amenity. 

I have a size-able portfolio that is a mix of commercial and residential.

The town I am in is rather small 35,000 but has a good sized college.

I don't analyze properties, I analyze markets for find the correct strike price, then strike when that number comes up on a property within that market.So, depending on the size of the market I'm analyzing, if I had to guess, I would say anywhere between 1 to 1000. 

From the research I have done, it seems the max size for an ADU is 1200 square feet.