When you have your big list of tax delinquent properties, filtering the list or trimming the list of properties for purchase follows. The guideline is simple: just focus on properties where the owners will most likely sell to you for a minimal price. Filtering can be done through review of documents (e.g. assessment, treasurer, tax rolls, and maps) and actual visits to the properties. Either approach can be taken first but document reviews is more efficient. Only visit the property to gather more information or validate findings of the document reviews.
Under document reviews, check the land value first. If the tax delinquent property is worth $20,000, automatically drop it from the list. Also drop the land that has huge tax liabilities. Also, check land uses and environmental elements to, further, filter your list. Drop lands even if they are cheap yet located near an industrial zone. If land is cheap but is flood prone, drop the property. Property should be, at least, be on a flat land, flood-free, far from the tornado, typhoon and quake lines, reserves and protected areas. More importantly, properties should not be brownfields, as classified in datatree.com or dataquick.com, or free from critical environmental concerns.
Visit the property though because information from maps, assessment, treasurer and tax rolls can possibly not be true or outdated. Owner of the $20,000-worth property might just sell it at $5,000. A flood-prone property might just not be flooded anymore after the state developed a sewerage system or repaired the drainage system. Least, the flood-prone land might just be flooded once in a year. A property on a hill might just have perfect geological characters (no erosions, not within fault lines, etc.).
These considerations are important because human settlement should foremost be in a safe location and not devastating to the environment like wildlife, forest and water reserves and natural parks.