Please advise, I want to start on the right foot. 

I'll go first: I don't have an exact count but my PM tells me it's 20+ in a 1,000 sq foot townhome.

For example, most new constructions are costing around $250 a square foot whereas turn key is costing approximately $300-$350 a square foot. 

I think the best strategy is to get your foot in the door with the least amount of risk on the first deal, which could be a turkey single-family or, depending on the area, a duplex.This way you can start building those relationships you need and figuring out the systems you will use at scale. 

Sell a property or another passive investment.While options B,C,D may allow you to "cash" your losses, you could be shooting yourself in the foot.

I would speak with a good real estate attorney and make sure you are on solid footing.

It is true that you may miss out on this home, if sold this week, but also consider the Joy Of Missing Out (JOMO) by not overpaying for this property and also footing the bill for many of the potential repairs required for a 120+ year old home as mentioned above by the previous commentors.I would encourage you to take a contractor with you to the open house or someone with experience in this age of home to give you some insight into the property and what may need to be addressed in the first few years of ownership. 

The 770,603-square-foot, 10-building portfolio traded for $144.5 million, or $188/SF, at a 6.25% cap rate.

Say 20x30 10 foot roll up door and a walk through door?

But a good place to start is by figuring out what a rough price per square foot cost would be in your area for a unit turn, cosmetic, and full gut renovation.Here in Chicago unit turn might run $15-25/SF, Cosemtic $45-65. and full renovation $90-$110 of interior renovations only.Hopefully this at least steers you in the right direction.