Adverse possession cannot be used to take government land. Even if you could, by asking if you could buy the lot, you would likely fail to prevail on the use of the land that is hostile to others and open and notorious to the true owner.
While this is legally permissible, I would caution you with regard to retaliation by an upset tenant and their subsequent arguments to a housing court judge that you are interfering with their employment, their ability to earn income and, their ability to pay the rent.
If you have title in your name, he will need to bring an action in court to prove that there was some agreement relative to the vehicle and that the vehicle was pledged as security for payment of that agreement. He cannot self-help by taking the car without a court order which appears to be unlikely unless you admit to the existence of an agreement.
In most cases, verbal contracts are every bit as enforceable as a written contract (just harder to prove) but this is not the case for your fact pattern. The statute of frauds requires that any contract dealing with an interest in real estate be in writing. There are exceptions, like an admission by the party or a written confirmation.
Contact the Minnesota Department of Revenue to be certain but, since you are operating a service primarily, you should not have to collect sales taxes. If you sell products into your clients, you will have to collect sales tax. If you are looking for a business start up attorney, feel free to give me a call.