However, you've come to the right place to ask opinions on what would be most efficient given the few things we know about you. 

It also creates efficiencies, compared to a collection of smaller investments.  

Then my pro forma with market lot rent & efficiencies has it back at about $2K/mo cash flow.I'd have it paid off in under 17 years and cash flow in the mean time.               

In a smaller multi-family that is already fairly efficient it might be difficult to move the needle.

After purchase I installed new flooring throughout, repainted the entire home, updated lighting to LED, updated master bathroom, replaced old furnace, air conditioner, and propane with a energy efficient geothermal system that heats, cools, and heats domestic hot water.

It really depends on the layout of the building, etc.If the plumbing isn't an issue, I definitely recommend the high-efficiency, direct-vent gas boilers.

I know speaking with contractors is the most efficient way to get accurate costs but it is too preliminary to do so.Any websites, softwares, books, etc to find some general costs that I can base off of?

Make sure your ARV lands in the biggest pool of buyers so you have the highest chance of exiting your project, efficiently and effectively.

What are some recommendations moving forward to maximize the efficiency of the rehab to get the property ready for prime buying season?

The majority of great deals I've found, the majority of the capital I've raised, and the reason I have been able to be so efficient with my work is because I have spent much of my time forming life long relationships.2.