Do keep in mind that the price of building materials, especially lumber, is increasing every week.

Instead, go to the local lumber yard.

building cost for single story 2 beds, 1bath ,2 or 1 car garage unit example our area ( between $100-$125 sqft /current lumber prices ) cost of building (not include land) of 4 plex, rent is about average of $1 per sqft/M Any feedback is much appreciated

Rough cut lumber is common here in the 50s which is awesome IMO as the homesteads were converted to suburbs.

I received an email from an apartment owner saying that he is planning to unload several of his properties in the upcoming months and he plans to take advantage of the 1031 tax code.

HelloOregon leads the nation in lumber production and in developing a mass timber industry - more info on mass timber in Oregon here https://oregonforests.org/mass...Are others seeing growing in interest prefab and in "impact" investing in real estate?

The shortage of inventory is causing prices to rise, the shortage of lumber and other build materials is causing build costs to rise, and inflation has become a large downhill snow ball. 

See Google page on Dissolving HOA in FloridaOn the other hand, real estate prices are getting so sky high that it's easier to unload a single condo than a doubly expensive duplex.

Any investor-friendly (good pricing, contractor cash accounts with discounted pricing, etc) supplier recommendations for various materials (flooring, tile, stone, countertop materials, hardscaping, lumber, windows, fixtures, roofing materials, etc) is greatly appreciated!

Depends on when he bought the lumber of course.