DR,
Firstly, welcome to the PU!
Secondly, and more importantly, thank you for your military service. Guys like you (and my own brother) afford us the freedoms to even have this discussion. Thanks!
As far as your question about what type of AK to go with, it can be hard to answer, simply because there are SO many of them out there. Moreover, some countries/models will capture a higher price due to quality and/or the rarity of the rifle.
I will say that from what I have read, heard and seen, the Arsenal AK-47 rifles are some of the best "modern-day" AK-47 rifles made. They use a Saiga receiver, which are made in the original factory (Izhmash) that the AK-47 was literally born in...and they are still produced there today.
Arsenal does a high-quality job of taking a "sporterized" Saiga, and converting it back to the original AK-47 configuration, including adding an accessory (i.e. bayonet) lug, and a muzzle brake, as well as the standard AK-47 handguards.
BTW, all Saiga barrels are chrome-lined. So, you'll get this with your Arsenal AK-47.
Bottom line, the Arsenal line of AK-47's are a great option to consider, especially if you're simply looking for something that's "turn-key"...buy it, shoot it.
As far as the difference between the SGL-21 and the SLR-107
F, the SLR-107F has a folding stock (hence, the "F" in its designation).
If you're going to put your rifle into a bullpup stock, you might as well get the SGL-21 instead. Plus, you may run into some "issues" with the side folding mechanism interfering with the new bullpup stock. Not sure about this, as I haven't seen the rifle firsthand...but, definitely something to consider.
BTW, I think those Center Balance Systems bullpup stocks look real nice! And, while I'm not into bullpup designs in general, the CBS stocks definitely have me doing a double-take. I especially like the ones for the SKS rifle.
Anyway, if you decide that you want to go with something else, G-broker is a good place to look.
I hope this gets you started. Good luck!