Before you get too far, I would go talk to every local bank and credit union in the area you want to do this. 

The reason being I have to build up my credit score before I can get approved for a loan. 

The answer is yes, 2nd lien DSCR loans exist and more than one lender can do them, but you would probably have to up your credit score to be able to get up to a total of 75% ltv between 1st and second lienOtherwise, you would have to get lucky on a favorable appraisal.

•Continue building your Paydex Score (80+) and Experian Intelliscore.4.Leverage Credit to Access Funding:•Once you’ve established some business credit history, work with a business funding specialist to secure $50K–$250K in lines of credit.

If you choose not to show up, he would almost certainly be awarded a judgement.Of course, you could choose not to pay the judgement, but then he can send it to a collections agency and your credit score will be trashed.I don't see a scenario here where you get through this cleanly.  

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

if you're a few years out before starting in the US with RE Investing, the best thing you can do is work on your credit and safe money and start establishing business credit with banks and credit unions here in the USenjoy Panama, and good luck

This was my first investment property and I used a loan from my 401k for the down payment and also used a local credit union for a 10% down payment on it when I bought it.

Your best bet for that size will be local banks and credit unions, good luck.

We have credit scores over 730 and have been "soft approved" for around $750k.