I'm a real estate investor (also a Realtor, but that's a different "hat" I wear) who makes lease-option offers on homes. So, it's possible/probable I do the same thing as the company that's made you your offer.
Most such operations are very small, sometimes an LLC with just one or two members. Some are bigger--maybe half a dozen people. And most tend to be geographically local. For instance, I focus on Northern Virginia. That's it. Others will pretty much focus on properties within a 10-30 mile radius. That's not an absolute rule, just a generalization.
With that in mind, no I don't know Dun and King. To check them out, ask for several references. Ask for references from at least two other homeowners who've signed with Dun and King on a lease option. Ask, also, for several business references--say an accountant and a lawyer. Even one-person operations should have an accountant and a lawyer. And a real quick tip: Look at D&K's business card. Then log onto VisaPrint (http://www.VistaPrint.com). Do you see that card design there? Now, that's not a deal-killer. But most beginnng investors use VistaPrint, and they go with the standard card design. (Some go with the free cards that have the VistaPrint contact information on them.) I wouldn't go with anyone who's using the free VistaPrint cards. If they can't afford $25 for their own business cards, they probably won't be able to meet more significant financial obligations.
Take the documents that D&K have provided to a good real estate attorney for review. Now, let me caution you that even so-called "good" real estate attorneys may not have experience with lease-options. When I was getting my documents reviewed, I encountered several attorneys who weren't familiar with the concept, or had other odd issues. Oh, and if D&K won't allow an attorney review by you, don't go with them. Whenever I make an offer, I encourage them to have my documents reviewed by their attorney.
Properly done, a lease-option (and I'm guessing that D&K is actually proposing a so-called "sandwich" lease option) can be a win-win for all concerned. You, the owner, will receive a steady cash stream and (as they're usually structured) relief from the maintenance and repair headaches you'd run into as a landlord. If it's a sandwich lease option, D&K will receive a slight positive cash flow through the term of the lease-option and a more substantial payoff at sale. And the tenant-buyer will have the opportunity to purchase a house, clean up his/her credit, and amass option credits...something he/she couldn't have accomplished without a lease-option.
So: Be careful. Have a lawyer review the documents. Ask for references. But lease-options can be a good technique to sell your home when conventional methods haven't worked.
Hope that helps.
Sounds as if you are talking about a lease-option. I am hoping you are being represented by a professional Realtor. You would proceed just as if it were a sale or a rental. they sould provide you with a financial statement for you to check out i.e. credit report etc. Is the company doing the lease option or an individual? Good luck.