There are many variables to consider and you should consider a SFR to start or LP syndication to start.

@Jacob PerezHi Jacob- do you have any lending connections for Canadians buying in the US?

Garage sale corporations, created factories overseas, leverage hostile take over, got into lending business then lost billions.

Look up their NMLS or real estate broker license number for a good starting point that'll weed out 90% of the scammers.For NMLS, it's centralized: http://www.nmlsconsumeraccess.org/ Some private/HML type lending, depending on state, will be under the auspices of the state real estate broker licensing entity.

The last one I bought needed a lot of work (conventional lenders wouldn't lend on it as it wasn't livable, so that took out a lot of competition).

However, when it comes to retirement account you should consider passive investment options such as private lending, investing in a syndication or private placement, buying a trust deed or investing in a note fund. 

A brokerage is fine if your non-traditional investments will not be overly interactive - such as the occasional note, syndicated deal, crowdfund, etc. 

So far I'm looking at Visio Lending and Lima One Capital.  

Can you lend to someone who already has multi-properties to learn from them?

@Gene AltobelliHere's some I learned and intend to implement:Create an LLC and have the LLC lend you a mortgage on the property you arereceiving.The reason why this works is because instead of you needing cash or receiving a cash out loan, we are now refinancing a loan – your loan.