The catch is that to keep the 0% rate, you will likely be required to make minimum monthly payments on time every month, even during the 0% introductory period. You need a clear plan for repaying the full amount you borrow before the introductory period ends, or else you will have to pay interest on the remaining balance, usually at a much higher rate.
Interest rates. The less interest you pay, the more loan you can afford. An adjustable-rate mortgage (ARM) is one way to lower that rate, at least temporarily. Because lenders aren't locked into a fixed rate for 30 years, ARMs start off with much lower rates. But the rates can change every 6, 12, or 24 months thereafter. Most have yearly caps on increases and a ceiling on how high the rate climbs. But if rates climb quickly, so will your payments.