As you can see from garthchapman`s response, the longer the amortization gets, the less benefit you get in cash flow - the amount of interest you pay increases dramatically, and the amount of added cashflow (because of lower mortgage payments) decreases considerably.
So it depends on how much cash flow you require/desire. But using garth`s example, I would say if you really need that $68 more per month for the deal to be a good deal, walk away and find another property.
I have used the 40 year amortization rather than the traditional 25 on my most recent properties, and it has helped get the properties cash flow quickly. What bugs me is looking at the amortization schedules side by side. The amount of interest paid on the 40-year is dramatically increased! And the amount of equity you put on the property in the first five years is dramatically decreased.
Anyway... also: a good mortgage broker is one of the best things to happen to me a few years ago! You need to find one as soon as possible. Running from local branch to local branch to find a loan gets old very quickly.