If the IRA is purchasing the property, then a non-recourse loan will be required. You cannot put a personal guarantee on your IRA's debt.
Most lenders offering such loans, including North American Savings Bank and First Western Federal Savings require 30-40% down and 10-15% cash in the IRA for reserves, depending on location, age of property and other factors.
The use of debt-financing in an IRA will incur UDFI taxation. This is not generally significant, but certainly something you want to review with your licensed tax advisor before moving forward.
My experiences has been that traditional lenders such as banks will provide a non-recourse loan on a 60 to 70 percent loan to value ratio. Visit here for a list of lenders to both IRAs and solo 401k plans.
If the the investment is 2-4 units, generally 40% of IRA funds is required.
If the the investment is condominium, generally 40% of IRA funds is required.
If the the investment is multifamily, generally 40% of IRA funds are required.
Typical Loan Terms Available from Banks Specializing in IRA and solo 401k Non-recourse Loans
-10 year fixed
-20 year fixed
-25 year fixed
-$50,000 to $55,000 minimum loan amount is the common figure.
Do be aware that with a debt-financed property in an IRA your IRA will need to file tax returns and pay tax on the debt financed portion of the property.