increase my down payment with my HELOC? or not?

[Jeffrey Hanson]

hey i am under contract on my first investment property, currently a 4/2 & 2/1 4600sqft duplex with a pretty easy conversion to 2/1, 2/1, 2/1 triplex.

my wife and i are going to live in the bigger half and rent the smaller unit to take a chunk out of the mortgage.

a family friend is fronting us the cash for the sale so we can close before the holidays, but i am taking out a mortgage to pay them back and using my existing HELOC for the down payment. i paid $120k cash for my house in VT in 2015, and in covid crazy times it was appraised at $180k for the HELOC. we are keeping the VT house, maybe airbnb or just rent long term. Anyway I have about $135k available on my heloc. sale price on the duplex is $279,000, so 20% is about $56k. the house is perfectly livable as-is, but at $60/sqft and properties on the block selling for $140/sqft i think there is room for some forced appreciation.

the building is currently a three-floor duplex townhouse, one unit is the ground floor and unit two is the top two floors. all floors are basically the same floor plan. the second floor has a kitchen/laundry stacked above the first floor kitchen/laundry. on the third floor, the room is in the same place and empty, but utilities (water gas electric) are already run and in place for adding a third kitchen/laundry, so conversion to triplex would be simple as installing cabinets and appliances and building an exterior staircase up to the second floor rear deck. the three 2bed/1bath units would rent for $1000-$1200 a month each, conservatively. we would have to find somewhere else to live for that to happen, but that is our 1-3 year goal anyway. 

would it be better to make the bank loan smaller with a bigger down payment from the HELOC? currently HELOC rate is at 3.00%, down from 3.99% a month ago when the 6 month initial lock ended (i have over 9 years left on my draw period). now the rate is variable. the bank is hinting that the rate on the mortgage will be around 3.5% fixed. i don't qualify for traditional mortgage on my dti (but i do double my income with overtime and bonuses, and my wife will have a new income as well once we relocate) so our family friend is cosigning on the mortgage with me. the bank is writing it internally at 15yr with a choice between amortized or balloon. the building will need a new roof in about 5 years, and leaving $35k in the HELOC for that would keep me happy, so i'm willing to go up to $100k down payment if it makes sense. that would change my loan to $179k from $223,200, but also would tap any funds i might have for improving the property other than the roof, like fencing in the backyard and converting the third floor to its own unit. the new rubber roof would be more like $20-$25k but i like to be conservative.

i have heard of people using their heloc to pay down their mortgage principal, which i could also do, but it seems to be a gimmick, and even if not may it be easier to just get a smaller loan amount to start? the family friend wants no interest, just to be repaid when we can. lucky for us!

thanks for any and all guidance

jeff