The PT (aka TG) version of the Paselas are excellent tires. Highly recommended. However, the non-PT version of the Paselas are very prone to punctures, so make sure to get the PT version.
Marathons are bomb proof but slow as fuck, not worth it for commuting or fucking around. Continentals are memes. Avoid them.
I would look at Michelin City and Specialized Nimbus too, but you can't really go wrong with Pasela PT.http://www.treefortbikes.com/product/333222416387/>>1189573
26" wheels for sure. There are a few versions of 26" with different ISO measurements, so make sure to get the right 26" (559mm). 99% of 26" are 559mm.
5/6/7/8 drivetrains can all use 8-speed chains which are common in first world countries, I don't know how common they are in third world countries though. I would do what the other anon said and look at what those countries sell online.. if they even have online stores.
Folding bikes I would suspect to be a bit less durable. More moving parts (hinges) means more things that can break. Is it likely to be an issue? Probably not.>>1189476
Chains are generally cross compatible. However, the shifter, derailleur, and cassette need to match (interchanging brands is sometimes okay, but not always; and even within a given brand it's not always okay, i.e. mixing road with mountain) because different shifters pull the cable a different amount per index ("cable pull"), and different derailleurs move a different amount per amount of cable pull ("shift ratio"), and different cassettes have different widths between cogs ("cog pitch")
If you want to know the details, read this http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/>>1188298
Depends how big of jumps and how durable your rims are. Modern mid-level mtb rims? Yes, absolutely. 90s rims? Maybe not. Regardless, ride the fuck out of it, have fun, and if you fuck up your rims, get a set of modern rims and keep having fun.