Nov's SSG is not good value.
It is on paper, but the amount of QC issues it has makes it shit. Not worth gambling €300 on something that may be a turd and a customer service that tells you metal filings in your cylinder are completely normal.
Being a Striker user myself, I wouldn't recommend it. While it is really easy to work with and it's almost perfect for my needs, it comes standard with an utter shit brass cylinder which needs to be replaced right off the bat. It's quite limited on upgrades too and some are really expensive (EdGi NEMO looking at you, and no, the Ares clone is shit)
I may suggest the Action Army T-10/T-11, it's more expensive than VSR clones (roughly on par with TM) but comes with a nice 90° trigger and piston. it's at least €100 saved on a new trigger box. Plus it looks dank as fuck.
As far as barrel lenght, stay on the 300mm mark, makes the whole package shorter (easier to maneuvre through dense vegetation) and can be used in conjunction with the full cylinder volume to Joule Creep heavy weights (something the Striker can't do unless you HPA it or drop €400 on a nemo)