How do you amortize/analyze a deal with a HELOC for downpayment?

[Michelle Reid]

On a 415k purchase of SFR.

I'll be using $30k cash and $53k equity line on my primary home.  

The line is 1.99% for 1 year and prime + 0 thereafter.

Interest only for 15 years is an option.  I would not use that option.

Would you amortize over 15 years which makes the payment much higher, or use 20-30 years hoping I can refi at some point.  Would that be too risky?

I'm using a 5% interest rate assuming prime will be there soon.  Difficult to plan long term on the rate.  

Am I missing anything?

Thanks for any help and insight!