After researching, we found that converting a single-family home into a duplex could cost around $200-300k.

Under Reason for Amendment it says, "300 Apple Street is located on a residential street primarily consisting of single-story detached dwellings zoned TN-E (Traditional Neighborhood Existing).

For quarterly payments, that depends on what your accountant advises.

However, as of January 2024, they haven't written a single ticket or enforced their STR rules in any way. 

Over several years, I have worked hard to save up for a down payment, and I currently have $200k set aside in a money market account ready to be put to use.

I currently have new quadplexes in Jacksonville, Fl  along with duplexes and single family homes (new) around the Tampa area.

The Assessments levied by the Association shall be used exclusively for payment of Common Expenses including, without limitation (i) the upkeep, maintenance and improvement of the Common Areas and other maintenance obligations set forth in Sections 10.1 and 10.3 below; (ii) maintenance, repair, replacement, and operation of rights-of-way and easements within or immediately adjacent to the Project (e.g. landscaping and sidewalks within the right-of-way of adjoining streets) to the extent that such actions are required by government entities or deemed appropriate by the Board, (iii) promoting the recreation, health, safety and welfare of the Owners and other lawful occupants of Lots within the Property, (iv) payment of property taxes for the Common Area; (v) insurance premiums; (vi) legal and accounting fees; (vii) management fees; (viii) charges for utilities and other services provided to the Common Area by the Association, if any, and for all utility charges paid or incurred by the City and reimbursed by the Association for street lights in the Common Area; (ix) costs to maintain the entrance features, including any electricity, landscaping and irrigation expenses associated therewith; (x) the performance and exercise by the Association of its rights, duties and obligations under the Project Documents, and (xi) maintaining any Stormwater Control Facilities located on portions of Common Areas to the standard required by the governmental entity or agency having jurisdiction over such areas.

@Ozgur CebeI recommend finding an accountant specializing in real estate taxation.

If in doubt about either of those 2 items then I believe an accountability group may be a better option.  

We've actually done a few this size as all single asset loans for clients who wanted a full 30 year term.