This all depends on your risk tolerance.

They are wanting to stay under 150 and put a little work into the house, maybe a little more depending on the deal.

It does varies depending on your market. 

I have over $200k equity in my primary and I will be taking out a HELOC to fund my first deal but looking ahead - I'm not sure how long we're going to hold on to this house so I'd like to focus on building a business line of credit that I can always depend on. 

Usually, you can do a cash-out refinance after construction, but it depends on the lender.

It depends on how the current deal is performing.

Depending on your personal life and your other properties, during the 5 year period you could live in 3 of your properties working towards this tax reduction.

I've looked at neighboring properties that vary in size.

It’s going to be highly dependent on your situation, assets, etc.

Depending on what brokerage you join you will have monthly office costs, and pay a percentage of your commissions to the brokerage.