Both will allow me to see where the Sellers numbers compare.  

Generally a HELOC can get you a down payment for the property, but generally not a whole property, it would probably make more sense to get the financing for any rental traditionally as it will probably have more favorable terms. 

On a related note, I was planning on traditional lending with a 20% down payment.

Hi @Bryan Clark, I am a novice compared to most people on BP. 

Not as good of terms as traditional mortgages, but still leverage ability.

Get educated on the local market, look at prices, cost per sq. ft. and what finishes, lot sizes, etc. are so that you're comparing apples to apples.

I am unable to use a traditional cash out refinance due to unquantifiable DTI ratios.

If you and the property qualify, a traditional mortgage is likely to be the cheapest money you'll find. 

Loans become more difficult to obtain, for the simple reason that the interest rate is a factor when it comes to approving mortgage loans – just study the formula of monthly income compared to the mortgage payment because lenders use your debt-to-income ratio to figure out how much of a loan payment you can handle.

So dont the lowered sales price affect all other comparable sales in the area?