OK looks like you've done some basic legwork. And enthusiasm will make up for a lot. Here's some unsolicited advice form a potential guest that I have gained in many years of travelling (I still go backpacking even at my age). Please feel free to ignore it or use it. These may be obvious to you, so apologies in advance if I'm insulting your intelligence. But you'd be amazed how many people seem unable to think these simple things through before they commit serious cash.
Like any start up business, you'll need to concentrate on cash flow. If you have a choice of building 20 rooms and a bar, start with the restaurant/bar and add the rooms to it. The highest margin you'll likely make is going to be on selling alcoholic drinks. I've seen people want to build the 'perfect resort' from scratch so they build the rooms first. But then they could not get any guests until the entire resort was finished and went bust before they could establish themselves. Also if you add rooms as you go you can tailor them to the guests you are actually getting (do you need one more cheap 8-bunk room or three more mid-priced twins?) A Swiss guy in the Philippines I recently stayed with built his dive resort this way over 5 years. As for food, does South America really need another pizza restaurant?? Use some imagination please
I'm pretty convinced that an existing business will be too expensive for you to buy. The value creation is when you establish the business and get the name known. People looking to sell are usually 'retiring' and want to get as much out of their investment as possible. They're usually not in a hurry to sell either.
Get your hooks out into the local community asap. I saw one African/Belgian guy returned home and built a luxury hotel. You'd think the locals would be happy that there was a source of income on their doorstep. They weren't: that source of Western income turned their daughters into prostitutes. Caused him huge problems in the long term.
Like any business, you need a marketing plan. Make sure people know where you are. One of the best source of info I use for booking backpackers is other backpackers (including customers, owners and staff.) I rarely take a Lonely Planet type book any more. Waste of space compared to word of mouth IMHO.
People need to be able to get to you. Make sure you are on a recognised 'route' because it is amazing how many people travel along the same logical path, like a major bus route, or the "Inca Trail". Look for example for the local equivalent of a travel organisation like Green Turtle / Magic Bus in New Zealand, Baz Bus In South Africa. Consider even offering free accomodation for the bus driver on these routes if your place is big enough. Alternatively make sure the local taxi/ moped drivers know how to get to you. Many times I've been told downright lies by some driver who has a friend in the business that my intended destination is closed / has burned down / spontaneously exploded etc. A few free beers/ preferential rates for locals is appreciated. That's life. There's the local price and the tourist price. Every bar needs its locals.
Go out of your way to ensure that other backpacker hostels upstream and downstream on the route know you personally, and swap advert flyers with them. In many places there's even a service that does this for you, but it doesn't negate the need for personal attention to what you might actually consider as your natural competitors. In a "new area" you need to work hard and club together to create that buzz to establish the whole route for the benefit of all hostel owners along the route. Remember that your hostel or bar will be one stop in someone else's much longer tour. Try to make it stand out rather than aiming at building another "hostel-bar-pizzeria-etc with the rest."
Make sure your hostel or bar is always clean. Doesn't matter how cheap or expensive a place is, people expect to be able to use the kitchen of have a pee in comfort. Make sure you have your fridge etiquette clearly in place. Build secure storage in each room. A good solid cupboard that a backpacker can put their own padlock on. Similarly, keep the bedding simple. Sheet sleeping bags so they are easy to change and hygenic.
You also really need a unique selling point: "A beach bar" on its own just simply isn't good enough any more. "A beach bar built around an S-shaped palm tree on a beach in the Pacific Ocean with dancing on the sand to the latest European tunes at a full moon party" is something else entirely.
Another friend of mine runs a lodge with yoga retreats. She has a matching kitchen to offer a cleansing experience. Sounds hippy, but pulls in the new age women in droves. Another guy I know does horse riding. Would work well in Argentina: the whole Gaucho package. At the other end of the spectrum you could be a jumping off point for an established destination like a volcano trip/ waterfall/ whale watching etc. If you can also include your own (mini bus) trips to difficult to reach local attractions like wine tasting tours in Chile that'll pull people in too. But this is probably going to be a pretty crowded space already. The message is simple: you need to carve out your own niche.
As for locations: Mexico will be too expensive I suspect. I have friends who work & travel a lot in Chile. They swear by Argentina as the best destination for travel in South America. Peru has many hidden treasures and is up and coming. Another guy I met in South Africa walked all the way from Port Elizabeth to Durban (1000Km) and picked out the best beach on the way for the location of his hostel, so that's anohter approach to finding a place. I also know a guy who lives in Bolivia and he loves that. He dedided on the basis of medium large city, low crime, decent transport links, no diseases, climate, and the cost of a building plot. Doesn't narrow it down much for you I know. But be sure to take the time to work out what are your criteria for choosing somewhere and do your own research first before picking a location.
If you've got diving e.g. in Honduras, Belize, Costa Rica, you'll get a different class of guests. It takes a lot more setting up, and is a lot more capital intensive, but you can charge a wedge more.
Here's some links for you, so that you can have a look at their business approach and copy the best bits.
[broken link removed]
http://www.bbh.co.nz/
http://www.bazbus.com/
http://www.greentoadbus.com/
http://www.hostelbookers.com/
http://www.travelindependent.info/america-south.htm
(template business plans)