We can close in 45-90 days depending on the speed of the borrower.

This is probably a place where you go to one of the personal loan sites to borrow money or maybe a credit union. 

If/when you raise the rent, and depending on how much you borrowed to buy it, I would expect you could do a cash out refi to recoup some of your capital. 

Although I wouldn't suggest this and you don't need to do this, most lenders have a due on sale clause and this could be triggered (altough low probability) when transferring title out of a borrowers name. 

There is one exception to this rule for VA borrowers, but it requires additional inspections to confirm the home is functional after having been moved more than once.There are some additional guidelines or requirements for the manufactured home itself that us lenders might have to satisfy prior to closing.

Later you can transition it into a full time investment rental down the road by doing a refinance and lowering the rate at the same time.There are renovation loans out there that also allow you to borrower over a 8-10 year term to cover the renovations and again refinance to consolidate the reno loan when the ARV increases.

Bridge Loans are short-term (usually around 12 months) at a higher rate and fee to get a borrower from one point to another.

Note origination where the borrower owns multiple properties with little or no debt, needs a loan for an acquisition, but can’t produce proof of income sufficient to get institutional financing.  

My thought is to structure a deal where we borrow her parents equity from them and pay them in monthly installments once we get the apartments rented.

Then borrowing from your HELOC at 4.5% will save a significant amount of interest.