Good morning,
Florida Architect and land developer here. Sounds like you have a fun little project. Most of the time in Florida (depending on location) geotech reports are not done for single or two story buildings. Most residential construction (in Florida) is done with slab on grade or a stem wall system. That being said I agree with Nicola, reach out to the owner of the other duplexes and see if they have any reports available, it never hurts to ask. You wont have to worry about slope here in Florida. As for the proforma, construction costs vary based on location, availability to materials and workforce. Reach out to a local contractor with your basic plan and elevations and ask how much per square foot and timeline they currently have. To save time you can take pictures of the other duplexes and get the interior photos from the listing. The contractor should be able to give you a rough sf cost. Once you have the cost to build, your assumed cost of land, financing terms (monthly payment), and local rent. You can reverse engineer the equation to see if your land is priced right.
Reaching out to the neighbors may also be harmful because they may not know this parcel is for sale and since they have other duplexes they may be interested in this land. Or the neighboring duplex owner may also own this land, then ask yourself "why are they selling this piece rather than building on it themselves." In the current land market I can almost guarantee this parcel is over priced.
Hope this helped