Three different structures get called "being the lender" and they have very different minimums.
Whole loans, where you fund the entire note yourself and you're the named lender on the recorded mortgage, need the full loan amount plus reserves. On a 250,000 dollar loan you're looking at the 250,000 minus any holdback you release in draws, plus enough set aside to cover taxes, insurance, force-placed coverage and legal costs if the borrower stops paying. People who do this generally keep 10 to 15 percent of the loan amount liquid on top.
Fractional or participation interests let several lenders each take a recorded share of one note. Minimums are commonly 25,000 to 50,000, sometimes lower. You're on title as a fractional beneficiary, which gives you real collateral rights and also means you can't act alone. The co-lender agreement governs who decides to foreclose, extend or accept a short payoff, and majority-by-dollar is the usual rule, so read it before you assume you have a vote.
Mortgage funds and pooled notes are where you're buying into someone else's lending operation. Minimums in that world are commonly 25,000 to 100,000, and yes, pooling money from investors is generally a securities offering, which is why those come with an offering document and an accreditation questionnaire. Whether any particular arrangement is a security is a question for a securities attorney, and it's the sponsor's problem to have solved, not yours to guess at.
Your construction read matters more than most lenders can do themselves. LTV protects you against the market. Reading a scope and knowing the 40,000 dollar budget is really 70,000 protects you against the borrower running out of money mid-project, which is the far more common way these loans go bad. A half-finished house is worth less than the one you underwrote. Lenders who can price draw schedules and inspect their own progress have an edge that shows up in loss rates, not in the rate sheet.
