I've seen some lenders place limits on the number of loans a borrower can hold, but most don't.

Borrow $90,000 HELOC.  

The returns will likely be greater if you source the borrower and originate on behalf of the IRA.

What is the best way to show cash in hand in order to buy it when you don't have the cash and need to borrow the funds?

GC's are all very well prepared in contracts now post covid, so there will be job stops when there isn't $ to cover those job cost runs. 

I've seen plumbers and builders stop hiring because all the "young" people start the job and then don't put in any effort or stop showing up.Figure out what you want to make hourly, then start telling everyone and their brother that you need work, how many hours you are available, etc. 

You can stop right there.

If you cash out equity in a property, you are "borrowing" that money from the lender.

(I know, nothing is certain but just hypothetically) and all cash out loans are not created equal. if you have a lot of equity you can borrow up to 60% with the lowest rate possible, which would be close to a half point lower then borrowing up to 75% loan to value. 

The structure will depend on the down payment, quality of borrower, terms of note, your cash out of the note sale needs, and other factors.