They also have a nice backing of partner contractors that they have screened and vetted for quality. 

A general rule of thumb is 90 days at 90% occupied, but again, I would hold on to the seller finance as long as possible.If you are buying out the seller completely when you do eventually refi, yes, you will need to have some equity (whether that's your own, or another private investor / partner). 

Most wholesalers also get their houses under contract by agreeing to a cash purchase by either them or one of their "partners/investors".

Just sayin'...Ever thought about finding an investor minded realtor and partnering with them on the deal?  

Partner with people who have experience and learn from their success.

My tenant wants to move in his pregnant girlfriend, what is an acceptable additional amount to charge ? . I havent raised the rent in the almost 3 years he has lived there , he currently pays $1K/mo for a large studio...

I am currently in escrow since I am selling an investment property along with a partner.

My potential partner is a GC who provides superior rates and quick completion.

If you don't know how, might make sense to partner up with another investor that has experience doing it. 

Since you have so much lead time between now and when you want to purchase you next deal, I would spend it doing two things: Building your network, and learning your area.I would join your local REIA and go to all of the meet-ups you can to meet the players in your area and see if there is a niche that is under-served or if there are investors that you can start working with/partnering with now that doesn't cost you money out of pocket.