Get a contract -- probably you'll want a lawyer if you don't know what you're doing. It should allow you to cancel the deal for any reason if you are not satisfied during DD.
DD -- check
Infrastructure -- all utilities and their condition. Roads, lights, electric pads & pedestals, gas, sewer, water. Sewer & Water especially tricky. Get a professional to inspect. Is capacity adequate? Are all the sites & homes actually usable & working?
Legal -- all permits & licenses. Code violations? Legal property description matches what seller owns & what you think you're buying? Any encroachments? Easements?
Government -- check in with the police chief, fire marshall, code enforcement, etc. Is the city going to be with you or against you?
Get informed on local laws & codes. (Notice period for lease changes, for example)
Demand -- are you sure?
Phase I environmental for sure you will want this.
Seller's expenses -- are they for real? Get copies of the bills.
Seller's gross receipts -- are they for real? Get copies of the bank statements (to see that the income actually is coming in). It should equal # of lots X lot rent.
Leases? What do they say? Is anyone on a long-term lease (this is bad because you can't change it when you take over until it expires.)
How long have tenants been there and what are they "like?" Do you need to make changes?
Make a list of all changes you are going to make on day 1. During DD is when you ask all the questions that allow you to be the "expert" when you take over. You should know everything there is to know about everything by the time the DD period is over. Even things that are not "make or break" the deal should be documented during DD because the Seller is unlikely to be interested in answering your questions after closing. Like, "where are the utility shutoffs?" and "Who do you use for plumbing repairs"
There's probably more, but that's a good start.