Monthly joint income is 7X the rent and one credit score is 630 another person is: 712.

This is a highly personal decision.

You need to have enough money, probably in the $100-150k, amount to put the property under contract, and do the feasibility study on the property, Phase 1 Environmental, Property Drawings for roads, water sewer, and utilities, and have a completed budget so you know what your costs are.At that point you can go out and attract investors, I don't thing that developing raw land is necessarily cheaper than buying discounted finished lots. 

I use and outside company to screen and it's typically only $45 for me per person and I ask the applicant to pay the fee.

We closed on the property and the seller-financed mortgage together, under our personal names. 

Would it make sense to save up money to buy something cash, finance, wait for market to cool down, etc.

In terms of getting into apartment complexes and large transactions - it's much easier than you think.Real estate is filled with opportunities to deploy capital, but in the same time is filled with fast-talking snake oil men that will happily take your money towards a "fund" so you own a portion of a large portfolio... only to find out the fund's performance is below par, your money is locked out, and the only person making money is the fund manager from the management fee they've locked you into. 

Find the house, flip it, sell it, pay my own personal expenses.What would be a fair payout to a financing partner like this?

Personally, I would only purchase for STR right now on either very unique/special properties or on those that also work out as LTR.I am a licensed REALTOR who is investor focused.

I'm gonna tag @Elise Bickel Tauber in this post as she's definitely the right person for you to get in touch with in terms of investing in the Pittsburgh market - smart move by the way.