we can go up to 80% on a non-owner line of credit assuming good credit and a min of 75k.

However, many of the portfolio lenders will do asset-based lending provided you have qualifying credit and the assets (property) makes sense to them. 

The average tenant in Trenton City is going to have a sub 650 credit score, a lower wage or unstable job, etc.

I've been educating myself on real estate for the past 3ish weeks or so but I have about 10k worth of credit card debt should I try and pay it all off so I can get my credit score up and have better approval odds for my first investment loan (I would be using my VA loan).

That $15K would have to be enough for either a 3.5% down payment using FHA for example and lean heavy on the seller for a Seller concession (Seller credit) which goes towards closing costs.

I know that everyone wants to get the lowest downpayment possible, but I've run into an issue where some lenders either don't accept seller financing or they allow for a very minimal amount.

Build a Financial FoundationImprove Your Credit Score: A good credit score helps in obtaining favorable financing terms.Save for a Down Payment: Start saving for a down payment on your first property.

I have $500k in cash to invest as well as a credit score between 750-800. 

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

If the new buyer defaults on payments or otherwise breaches the terms of the mortgage, it could impact your credit and financial situation.Market Demand: Consider the market demand and potential buyer pool for properties sold under subject-to arrangements.