Consulting with a real estate attorney or financial advisor experienced in such arrangements might provide valuable insights into making this deal happen smoothly.

If a new investor comes in with a 300k purchase and a 200k rehab in a high end market where it will take 3 months to sell, a lender will not be so hot to lend a newbie in that situation.

Considering your situation, holding onto your current duplex with its strong cash flow and exploring refinancing later when market conditions are more favorable and you have built up more equity seems advisable.

To evaluate your situation you need to get a handle on the expense side of the equation

Bank Statement program will be best to acquire the property and just refinance to better terms when your financial scenario changes .

What would you do in our situation?

Timing played a role in my sucess as 2010 was basically the bottom of the financial crash (wish I would have been more aggressive back then :-)). 

In terms of yields, capital appreciation, and taxes.I was considering Dallas TX and Tampa Florida as of right now because I feel they're both strong markets and favourable tax situations.

I am specifically thinking in terms of a partnership and/or private money lending situation, but am interested to hear any experience whatsoever!

That is a tough situation.