I wouldn't buy a list. The database process is 4 parts.
Build a list (MHVillage has a good one and it's free to look at. Just hire a virtual assistant to move this data to a spreadsheet)
County Records (Parlay 2.0 is a great service for this. It is Google Earth based so I also like to have the virtual assistant drop a pin on Google Earth once they've gathered the owner records)
Secretary of State search to find owners of entities
Skip Tracing to find cell phones, land line phones, accurate mailing addresses
Buying a list helps you with step 1 but steps 2-4 are where you start having a database that you can do something with.