We bought it for $225k with a 3.125% mortgage and 1) are interested in keeping it as our first rental. 2) We are thinking about using some of the equity (maybe $50k or less as a HELOC to combine with existing cash for our next home down payment that we can convert to a HELOAN through our credit union).

We have some credit card debt we are hyper focused on paying down , and 34K in my student loans (not touching currently due to SAVE plan with the zero percent interest).

Most of my clients run their SFRs at a 25%-30% (excluding vacancy/credit loss) operating expense margin and most of the banks I deal with on their DSCR loans underwrite using 35%.

Also remember just because a loan may qualify as being assumable the lender will go through a thorough underwriting process to confirm you meet eligibility for the loan.I have seen a guru teaching people with poor credit to go after these types of loans and it’s unfortunate as many would most likely not qualify.

However, I've researched different ways of doing this, such as a home equity loan, Home Equity line of credit, and cash-out refinance.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

I ran some quick numbers and would estimate (assuming you have good credit) you could borrow around $415k-$420k, which includes funding 100% of the rehab costs.Have you calculated your expected profit based on selling after rehab and if you decide to keep long term? 

If the students do not have the income or credit to warrant the rent amount, I would require that they get a cosigner (like a parent or another relative that can vouch for them).

sometimes to reduce borrowing costs, I'll use a hard money loan solely for the purchase, and then pay for the rehab in cash or use a 0% credit card (I don't recommend that for new investors!)

This also includes a few credit cards that are used for business expenses.