Do you have a pre-approval or proof of funds ready?

These are done between the buyer (you) and Seller.The situation that is most common to find these properties are in a foreclosure proceeding, or pre-foreclosure. 

I had 2 or 3 called.Now again keep in mind I had zero interest in holding these as rentals and I ONLY bought homes that had significant equity for me.. mainly all pre foreclosures..

NEVER borrow from a 401K, you are taking pre-taxed dollars and paying it back with after tax dollars, you do not get to return the loan with pre taxed dollars, in addition, some 401K plans do charge interest and maybe even fees, and lastly if you get laid off for some reason, it is a distribution with all the "fun consequences: tax at the last dollar rate, and if you are under 59.5 there is also the additional 10% penalty.Personally I like the HELOC, however you can also sell stocks and do tax harvesting there to offset gains if that is an option

Then any rental income you could obtain from that property could be used as an income source for loan pre-qualification.

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.

Quote from @Miguel Garcia: If I look into investing out of state should I look to get pre approved by a lender in that area or would a lender in my area be fine?

The defendants researched and located properties that were in the pre-foreclosure short sale process more:https://www.justice.gov/usao-edtx/pr/self-proclaimed-short-s...18 U.S.C. § 1344 – Bank Fraud18 U.S.C.

It's probably going to mean paying points at closing, a high rate, and a serious pre-payment penalty (like 5 years). 

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