you're better off just buying a cyma ak for 150$ when they come on sale. A new/stock gun gives you a known standard to work from. I find people are selling mid and higher ranged guns for more than I'd be willing to pay for a used/parts/tech platform. Buying a working gun also gives you the benefit of it actually working, rather than someone's abandoned project that even if you do everything right, it still might not work.
Gearbox innards and principles are all the same, hey have some quirks between versions. 3s are easier to pull out and work on after testing to see if you fucked up or not.
A p90 is the best platform to learn on because it's literally 2 screws to drop the gearbox out, 3 screws to take the trigger off and the rest is contained. The cutoff is external and the motor is caged. If you get the KA one with the quick change, then you can take the spring out and completely unload the gearbox so shit doesn't go flying everywhere. The only difference in the way things work in a v6 vs a 2/3 is that the tappet plate spring is a compression spring, not an extension spring.
But not having to worry about shit exploding everywhere when you open the gearbox is a huge plus.
Look for a cheap, new m4, ak/g36 or p90, cyma, jg, ka, g&g plastic bodied if you have to if you want it cheap, that way you're not out a lot of money and you still have a backup gun, and if you fuck up, it's a cheap gun to start with. Lots of choice under 200 bucks.
Stay away from anything that has blowback, those systems are all proprietary and aren't standard to learn from.
I'll just offer this as advice if you're serious about learning:
It's not about just buying parts and putting them into a gearbox. You need to understand how things inside work and what forces are at play. It doesn't require a lot of special skill or tools, (getting certain things to fit properly might require tools) but to be any good at it requires some time and reading. If you really want to, you can do it on your scar, just be sure to read up on it so you know what you're doing before you do it.
I just don't recommend learning on a scar because it takes so much time to take it apart and put it back together in order to test your changes to see if you've done it right or not. the sportline scar is already a pretty low tier gun.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.