You must not be from Canada, and if you are Id just like to let you know that evike it a shit company. I cannot comment specifically on all your inquired
items but I will try.
Shims? Shims are shims in my opinion. I put systema ones into mine.
Metal bushings, sure why not. Just a forewarning, not all brands fit. Currently I have had issues with deepfire and systema. That is not to say that your mechbox is the same as mine.
Gears? Whats wrong with the gears in the mechbox now?
You can get that anti reversal latch if you want. Although I recommend this one for many reasons. E-ring to prevent the popping out of the latch, and the quick release notch incase your piston locks back, it helps save time instead of opening the mechbox.
G&P grease set is decent, although I found the cylinder grease a little runny.
Sorry no real opinion on the BBs. Heavy weight is better for foliage penetration, just make sure they dont have seems as those may damage your inner barrel.
Cylinder set? sure why not, it looks fine.
The spring? M130 sure why not, as long as your fps stays with in your field/playing areas limits you can pick whatever spring you like.
Honestly, not counting the cost of shipping the grease or the BBs. Your already looking at 170 dollars worth of parts for a gun thats worth about 150 dollars american. I should be one to talk though, I put a 160 stock on a 150 dollar gun.
The stock spring on that 416 will get worn in eventually and you will notice a drop of fps anywhere from 20-30fps. If you wish to replace the spring use anything you want, anything over an m120 and you may wish to go along with the gears you choose. Anything above that and you may wish to consider a torque up gear set with a halftooth piston set up. Also Id like to recommend a bearing spring guide.
My personal recomendation.
Modify v2 spring guide
Modify bushing or some form 6mm
Modify piston head
Modify cylinder head
A modify full tooth piston of some form
Modify antireversal latch
I choose all of one brand because it has been noted that to many mish mash mix of parts can lead to failure quicker. Using all of one brand may help in reducing that.
Just want bare bones?
Get some shims and new bushings with the modify anti reveral latch and modify v2 spring guide.