You're going to love motorcycling around Guatemala! Its a great country to explore and there is a ton of sights, awesome roads, trails, and culture. Check out my ride report link below to get an idea.
You can buy a bike and put in your name as a foreigner. You will go through some "red tape" but its something that is done quite often here. Myself and plenty of friends have done this as expats in the country.
I don't see the bike you posted on the streets here but there are a lot of very similar options available. Honda sells a CGL125 that looks like the equivalent to what you posted. http://www.honda.com.gt/motos-doble-...ito/cgl-125-tp
I will tell you that if you buy a new bike, getting the plates usually is going to take a while.. sometimes a month in my experience. If you stay in Guatemala this isn't a big problem, but if you are going to cross borders you will need a plate. So buying used might be another option to consider as you can put the bike in your name within a few days. Actually, here is a checklist for transferring a bike into your name that I put together for others:
The last thing I will say is that a 125cc is going to be pretty small to go 2 up, we have a lot of elevation changes here and depending on your size, I think you're going to want some more power.