if you have no gutters or downspouts taking the roof water away from the foundation its going to puddle .. and of course depends on the soils etc.. 

When was the house originally constructed and in what type of soil (embankment or natural soil)?

You can get someone to come out and do this, if you find them you probably need to do soil testing.

It can be leased out to farmers depending on the soil quality.

Nefarious folks may try dumping oil, chemicals, etc. on your land and contaminate the soil.

It is a 90 year old Florida house so not sure if the property sank into the wet soil over time and wood has rotted or if they just didn't build crawl spaces in those days. 

Also, you may have a little trouble if you're thinking of replicating your strategy from Appleton here - something you should know about Houston (and several parts of Texas) is that the weather + soil composition often leads to home foundation issues. 

Has it been soil tested? 

The former use as a auto repair could present some concerns and you should find out how the former heating oil tank was abandoned (removed, closed in place, etc). since you are converting this building as opposed to demo so you wont be excavating impacted soil, the principal concern would be potential for vapor intrusion since you're planning on converting to residential. this can be addressed with a system similar to radon system.

@Ismael Martinez - not sure if it's required to build an ADU in San Fernando, but a bunch of cities in the Bay Area require a soils report from a Geotechnical Engineer for new construction like ADUs.