I personally like Listsource because they have some of the best, most reliable data out there.There are a number of direct mail options out there if you want to outsource.  

I can't say for sure but the MLS data is just compiled from the public records and that is going to be how most of these list generators will get their data.

How can I best ensure that I can get something if her business goes bad but I still have an 8 year lease in my hand?  

At the end of the day, they are taking ZERO risk when you buy that house and frankly you look at the deal totally differently when you're the one at risk...as you should.I would carefully search out all my own comps and wrestle with the data on my own.

someone please correct me if I am wrong)  I am able to search properties by address, owners name, parcel etc and look at recorded data, and I am able to look up tax information and past sales info (to a certain point at which they began electronically recording them I believe).  

From this data, you can utilize a website bestplaces.net that will give you a breakdown of the percentage of homes that sold, in various price ranges, for a given zip code.

@Aj Bowman- in terms of the property's market value, this can be a tricky thing to determine with vacant land (a lot of the appraisers I've talked to don't even have a firm handle on how to do it with much certainty - because in many cases, the data needed to make the determination simply isn't available).That being said, there are several things you can look at to help get you most of the way there.

From this data, you can utilize a website bestplaces.net that will give you a breakdown of the percentage of homes that sold, in various price ranges, for a given zip code.

There are usually many times when there are no new listings in my area, so random checking throughout the year tends to give me better overall data than just waiting until it's coming vacant.  

Or, do you know of any sites where I can pull the raw data and feed it into a program like ArcGIS?