There's no legal minimum. The floor is set by fee drag and by concentration.
On fees: a flat custodian schedule around 300 a year is roughly 0.7 percent of a 45k account. Add a servicer at 20 a month and you're near 1.2 percent before you've earned anything. That's survivable on a 9 or 10 percent note and painful on a 6 percent one. The same 320 on a 200k account is 0.16 percent, which is why people say these accounts want scale.
On accreditation: the standard test is a net worth over 1m excluding your primary residence, or income over 200k individually and 300k jointly for the past two years with a reasonable expectation of the same. For an IRA the sponsor generally looks through to you as the beneficial owner, though how a particular offering handles that is a question for the sponsor's counsel. A 506(b) offering can take a limited number of non-accredited investors who meet a sophistication standard, so those exist, but they can't advertise, which is exactly why you don't see them.
At mid forties the realistic paths are a fractional interest in a note alongside other lenders, a smaller-minimum mortgage fund (25k minimums do exist), or holding the account somewhere cheap and adding to it before deploying.
One thing worth deciding early: interest from lending is exempt from UBIT, so it compounds inside the account without a tax filing. If you drift toward a fund that does fix-and-flip operations rather than lending, that income is treated as business income and can trigger a Form 990-T filing at just 1,000 of it. At your account size that filing can cost more than the tax.
