That's some nice kit and useful information you provided me with!
At first, I was thinking about the UMAREX MP5. Either the Elite Force H&K or the Heckler & Koch HK MP5 A5 Dual Power Spring, I don't know which one is better.
https://evike-europe.com/78863.html - Elite Force H&K
https://evike-europe.com/2-6311.html - Heckler & Koch HK
As for the front grip, I believe I can use a Magpul AFG or something 3D printed.
The only problem I have is the extended barrel, which might not be so easy to find.
But thank you for the help, I will consider your advice!