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.

A couple of tips - you may have thought of this already...1) An architect can help you with both the fire wall issue, and the efficiency ratio (IE: how much is actually rentable vs how much goes to corridors, etc) Totally worth it if you can increase your efficiency ratio.2) Contact your local utility company to see about engineering assistance and/or $ assistance for installing new lighting and/or HVAC.

That’s cost efficient.

All appliances ($10000),  New HVAC x 4 - as old furnaces used for church are not efficient and were huge.  ($18500).