They have told me that assuming credit, debt, etc check out fine that I will have enough equity in the land to make the downpayment for the cabin that I want. 

The only things that matter are credit, down payment, reserves, and DSCR ratio! 

I have about $400,000 in available credit on my credit cards.

I'd love to have a small line of credit on the property before the end of the year to cover any other repairs or upgrades that will need to go into the property.

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

Those "good contractors" usually have the lines of credit or means to pay for supplies up front if they know they will be paid on time and are not desperate for a deposit to go get the supplies.

Either way its good to buy end of year as most builders and sellers are offering seller credits or builder credits to use towards closing costs or to buy a rate down to help get a lower payment.Even if you wanted to buy a strater home like a 2-4 unit as a primary again you only need to put 5% down. 

Perhaps slightly higher from an alternative lender but definitely not from a credit union and expect much higher fees.

They are business CC but I don't use them anymore and would rather use cash/debit cards.

@Otis Clayton You say "I am at the point that I want a partner with some cash and credit to refi the deals I get from marketing and talking to sellers"Based on that you are looking for someone who has cash and credit, wants to invest/benefit from real estate but does not have time to find deals.Look for someone like that, be clear on their role and your role.