Should I stick to hard deadlines or show that I am willing to be flexible? 

The sellers were clearly motivated and were wary of FHA loans, being able to do conventional financing eliminating PMI and offering flexible closing date reassured the owners and they jumped at our offer.

I'm flexible on what I'm looking for but primarily what I would like is to fix up a duplex. 

The more open and flexible you are, the better chance of having an investment asset structure that allows you to do what you really want to do.

Alternatively, if you find a piece of land or teardown, you can consider what will sell in that particular neighborhood, and work backwards.My thoughts are that the market is really looking for flexible housing options.

Single member IRA LLCs are much more flexible and cost effective over the long term.With that in mind, if you can make investment A with your IRA LLC and your wife can make investment B with hers, then that is clearly simplest and cleanest. 

I’m pretty flexible.

The reason being is that you have the flexibility to geographically spread out your risk.

this one is also free, but has some more flexibility and it's mobile. the late fees is one part, also you can rent out by room and charge the tenants separately. 

I'm essentially going to rent (pay for) 2-5 bedroom units from local landlords at full price, and then sublease those rooms out at a markup (I wouldn't charge tenants broker fees and I'd give them flexible leases to justify the markup, among other things).To the landlords, it would essentially be guaranteed rent from a trusted company, along with free tenant screening and property management (i'm trying to make it as appealing as possible to landlords, as you guys are generally hard to sway).