I set the Credit Score as 650+ negotiable in Zillow and I had two immediate applications with very bad scores. 

I have a few business card but my favorite is PSECU personal credit card .

Quote from @Geoff Prickett: Anyone recommend a great bank or credit union for working with Series LLCs?

I use the biggerpockets credit check.

I’d start by reaching out to local portfolio lenders (credit unions are great for this) or a broker (me) who does investor loans – they usually have products designed exactly for this situation and do a 2nd position that'll allow you to tap into the equity without touching your 1st. 

Purchase price is 210k ( plus a 6500 credit from the seller) rents I'm seeing in the area realistically are 1450-1650 per unit.

Fill out an application or complete credit/background check, no. 

Pay the line of credit and then you'll have more leftover to buy more. 

But generally speaking, managing your credit score is a common task for real estate investors - especially for those who make use of credit in the purchase and rehab of properties.

I recently did a loan for a 5 unit mixed use and had some success with getting a loan through a credit union.