One thing you probably want to do is to find out whether there are any zoning or sign ordinances applicable to the land you want to lease. In many places, new billboards are banned and only "pre-existing" or "grandfathered" billboards erected before the ordinance was passed may be allowed to continue.
You can consult with a land use attorney in the area of the land or contact the planning or building inspectors with jurisdiction (which may depend on the municipalities the land is sited in, whether the area is "incorporated" into a town or county, etc. Sign ordinances, except for those located within about 600' of an interstate highway which are categorically prohibited, are inherently local.
Otherwise, the purchase of the land, conveyance and leasing are "plain vanilla" real estate transactions which any lawyer or realtor can handle...you may not even need a lawyer.
One more thing: billboard lease contracts are "adhesion" form contracts, in other words, "take it or leave it" on the billboard company's terms, including rental rates. There's not much negotiation possible.
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Talk with the County before you purchase the land and be sure that what you are planning will meet local zoning and land-use requirements. You should be able to do that with a simple visit to the planning department counter.
If what you are proposing is allowed for that land, then after the purchase, as the commentor above indicates, the lease with the billboard company will be pretty much take it or leave - and not much is required on your part than to accept or reject those terms .
I am almost 100% sure that you won't have "zoning" problems because most of the zoning restrictions in Texas are done at the municipal and not county level. It's possible that you'll run into either a county-level restriction or some sort of covenant that runs with the land -- but it will be easy and relatively inexpensive to check for both types of restrictions.
As with all land purchases, you will want to do your due diligence and make sure there are no problems with your title, and make sure the transaction is done right and is properly recorded. It may also make sense to incorporate and have the business entity enter into the agreements rather than entering into them personally.
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