Patagonia is massive, windy, and entirely worth the long flights. If you head down in November or March, you catch the shoulder season. The weather is still decent for hiking trails like the W Trek, but you will not have to fight crowds of tourists for a spot in the refugios.
A trip like this usually runs about $3500 once you factor in flights, park passes, and gear. Instead of draining your savings all at once, we can break that cost down into monthly payments over a year and a half.
Just remember that 18 months is a hard deadline, not free money. If you have any balance left over when the interest-free period ends, the regular interest rate kicks in on the remaining amount, which makes the rest of your trip much more expensive.