Looks like a pretty standard DSA. DSA are known to be some of the best in the business.
If you really hate the flash suppressor, a smith should be able to remove it and install your choice of muzzle attachments. BTW: The barrel is already threaded to fit other devices sold by DSA.
I've heard good things about the ARMS mount. Another BTW: Upper and lower are used just like an AR. Upper holds barrel and bolt, lower holds trigger stuff and stock. The only difference is that the upper of an FAL is the serialized part, where an AR lower is the serialized part. The part you refer to as the upper is simply a cover, dust cover, topcover, etc...
It is a metric FAL, the vast majority of DSA FAL's are. Easiest way to check is to look at the mags. Metric mags have a small tab bent out on the front, inch mags have a larger tab spot welded on. There seems to be more parts support and aftermarket stuff available for metrics.
The gas adjuster adjusts how much gas is bled during a cycle, NOT how much is directed to the cylinder. That means the higher the number, the weaker the cycle. To check adjustment, fire a round, turn the dial up one click. Repeat. The first round that doesn't cycle properly indicates that the number is too high. Turn it down one click. This is the maximum setting that will operate the rifle. I recommend one setting lower just for reliability. Changing ammo may require a readjustment.
I just clean mine. Bore, bolt, internals, add some oil, wipe down with oiled cloth. Others may have more specific procedures.
Seriously, check out falfiles.com. Those guys have as much experience with FALs as people here have with AR's.
If you want to make it a folder, you will need to purchase a folding kit. This will be a new bolt, lower receiver, top cover, spring, and stock. The kit sold by DSA costs almost $400
To measure barrel length, close the bolt, and slide a rod down the barrel until it stops. Mark rod and measure. Must be at least 16 inches. ATF measures overall length to be the extreme length with the stock unfolded. Minimum 26 inches. Your state law may not agree. Always check local regs. If the barrel is over 16 inches, there should be no problem putting a folder on it, as the receiver and folded stock should be able to make up the other ten inches.
Welcome to the world of the FAL. 5.56 is nice, but sometimes you need a bigger hammer.