And use all of the proceeds from the sale in your purchases you'll defer all tax.This does give you some interesting options because you can allocate those proceeds anyway you want. 

Just make sure the town is ok with the land being zoned for multi-units, and look into construction costs, financing options, and permits ahead of time.

Most 1-4 unit real estate investors are using DSCR loans right now for any buy and holds while flippers lean on higher leverage bridge/fix and flip loan options with higher interest rates.

Also Cocktails and Real Estate at Patriots place.BP does have a local meetup section which usually has good local options.

So you could see about going to auctions and winning a "cheaper" bid on a property there, if you decide that's an option.

I have 2 options:- finance the construction pre-selling the apt, and take my profits (expected to be around 700K pre-tax); or- finance the construction with a 6-7%/year loan (collateral = my house), keep the 10 units, and once the building is completed in 24 months, rent them for circa 180K/ year gross.

While I’m open to both turn-key and fixer-upper properties, my preference leans slightly toward value-add opportunities where I can improve the property and increase equity or cash flow.That said, I’m also happy to consider turn-key options that provide immediate cash flow if they fit within my investment strategy.

Even in HCOL areas, adding an ADU typically does not make sense because developing a single small unit is the most expensive residential development that exists.Adding an ADU is a lot of work (probably more than a brrrr but with worse return), has poor financing options, typically adds less value than the cost of the addition, and detracts something from the primary structure even if it is just yard and privacy.Good luck 

There's a few mortgage brokers in the area I'm happy to connect you with do dive deeper into your options.

In addition to conventional loans and FHA loans, if either you or your wife are veterans, the VA also has a rehab program.I'd be happy to discuss my rehab experience both as an investor and as a mortgage broker, and at least help you think through your options.