A vacant land liability policy covers claims from someone getting hurt or causing harm on your land. A trespasser on a dirt bike, a tree on your side of the line falling onto a neighbor's fence, a hunter who lets himself in. You own the ground, so a claim can come to you even with nothing built on it. These policies are usually inexpensive, often a few hundred dollars a year for acreage, and many carriers will add the land to an existing homeowners or umbrella policy as an endorsement instead of writing something standalone. Availability and what's excluded vary by state and by carrier, so ask your agent to quote both ways.
On licensing: buying and holding land for yourself needs no license anywhere. Marketing or selling land on someone else's behalf is what triggers licensing rules, and those rules differ by state.
For data, county GIS and the assessor's parcel search are free almost everywhere and will give you ownership, assessed value, acreage, and usually a zoning layer. Paid parcel and sales platforms generally run somewhere around $50 to $150 a month for a single-user plan, and their value is mostly in comparing across county lines quickly. In roughly a dozen states sale prices aren't public at all, so check whether the subscription can actually show you prices where you're buying before you pay for it.
The line that dwarfs all of these is property taxes, and unlike the others it tends to climb as development approaches.