[Maestro]

Because usually you can deduct the interest from his taxes where the 401K loan can not be deducted and if you change jobs it has to be repayed immediately. But tax deduction on a HELOC today only applies to the primary resident, not a income property.

Because Mortgage brokers did all kinds of bad things with HELOC and letting people use them as the down payment in addition to the mortgage the government has put lots of restrictions on them.

Really you should talk to a tax account there are benefits of doing a mortgage on an income property. Also I hope you set up an LLC to buy the property and have it managed through that than directly by you. You may need to do some more home work on this.