the GSG-5 is cool, supposed to be the only way to get an MP5 for under 3 grand (in the .22LR caliber).
I considered the GSG5 too, my research indicates that its just a cool looking rifle and not much else. magazines are expensive and don't hold much (especially for California residents where I'm now), magazines are not readily available in any gun store, I have found its is accurate and reliable, but so far after about 5 dozen reviews to me its just a fancy and pricey toy. It looks cool and that's about it. If you think its worth the $500 then go ahead and buy it. To me it wasn't worth the $500. Just because its cool isn't worth spending $500.
a 10/22 with the tacticool stuff certainly will cost more than a GSG5. Mine does.
I got a used 10/22 carbine for $180.
I felt it was too easy to accidentally ding the wood stock, so I bought a polymer Archangel 5.56 stock ($160)
The stock has no built-in sights. I got a red dot holographic sight for $35. its an airsoft imitation of the real thing, it works actually quite well. I really wanted one of those red dot holographic sights people buy for the AR15, but that's like $500.
$40 for a mounted flashlight/laser, and $40 for a vertical grip.
10- 10rd magazines cost me about $150. But this is California, where everything costs more.if you are outside of California for $150 you will get significantly better value for your money.
If the 10/22 magazines looked like AR15 magazines it would easily pass for one.
the 10/22 is a better choice because not only is it popular (so if it broke it wouldn't be too hard to find parts or gunsmiths to fix it) you have nearly infinite possibilities of adding stuff to it. If you don't live in a AW Ban state like I do, You can get butler creek 25 and 50 rd magazines for not very much, I imagine target practice will be much more fun with 50rd magazines.
There are other tactical 10/22 stocks.for some odd reason all the other ones are illegal in California except the archangel, despite they all have the same "assault rifle" features. I would of bought something cheaper if it was legal. Tactical stocks start at about $100.
Another thing about the 10/22- its not $500 right now, since each part can be bought separately you can do that and it won't seem as expensive if you spent the $500 over time.
For the FAL well you can always buy one later, I think there's some cheap surplus magazines available right now, if you didn't get around to buying the FAL at least you did get a whole bunch of magazines cheap. You can get the FAL later then.
I don't know about making your own AR15 pistol, there's a Kel Tec PLR-16 pistol that's AR15-like and accepts AR15 magazines. I don't know the price as that isn't CA legal so I can pretty much forget about buying one of those.