Have you heard this thing in person? I find guns sound much different in person than in video. I've personally always been unimpressed by the sound of AEG blowbacks, especially lower quality ones made from thin pot metal. They always sound so hollow and "clacky" to me. There are no GBB SIG's btw.
You probably want to find out the OEM of this gun, Cybergun doesn't make them they only provide the licensed trades and branding. It will make it easier to know the manufacturer when trying to find compatible parts.
1) I use Mag brand mid-caps, they've always worked fine for me.
2) The SIG 551 uses the same length barrel as an M4, 363mm, any AEG barrel work. IMO, Prometheus EG 6.03
3) The SIG uses a medium type motor
4) There are aftermarket receivers for SIG's (Hurricane makes one) but you'll have to be careful, they may not be compatible with a blowback system or could require extensive modding. SIG owners much like SIG parts are somewhat rare and my guess is you'll have trouble finding someone who's attempted this before. You may have to bite the bullet and gamble or just be satisfied with the stock receiver. I've seen adaptors for SIG's to take an AR style collapsable stock but that was for real steel. I'm not sure if they exist specifically for airsoft. I'd be curious to find out though.
You should also buy a better hop rubber. For the cost they are one of the best investments you can make in your set-up. I've had best results using Firefly or PDI W-hold rubbers.
My first gun was an ICS SIG 551, I rarely use it anymore but I love the thing to death (tear). I paid way too much for it a couple of years ago but with prices today ($260) they're an exceptional value. The motor in the ICS SIG (ICS turbo 3000) is very loud, thats mostly what you hear when shooting it otherwise it sounds much the same as any AEG.
I find, myself included a lot of people place too much thought into their first gun. Someone who is willing to put a lot of time, effort and money into a custom project gun right off the hop is someone who will probably be around for a while. This is also someone who will go through many different guns over their airsoft career.
The advice I received and will give to you is, for your first gun get something that is quality and reliable out of the box. This way you can just get out use it and not worry about your gun keeping you out of the action. After you have a reliable working weapon then do a custom project. It's not uncommon for airsofters to own many different primaries, I have six.
So it's not what you wanted to hear but G&G makes probably the best SIG's on the market. Yes they don't make a 551 but my advice would be to get yourself one of those, a 552 or 553. Use it stock or throw a new hop rubber and maybe a new barrel in and you're good to go. I guess it all depends on how important the sound is to you.
< sounding the SIG signal