How do they perform, do they:
1.shoot straight
2. & have a reasonable fps, by reasonable fps I would say 250+ for the pistols & 285+ for the rifles.
If points 1 & 2 are achieved, then leave them alone internally, they're not broke so you can't fix them so to speak, & attempts to usually go one of two ways, the manufacturer balance is upset & they end up worse, or alternatively you chuck more than their worth at a "tech" (?) just to gain a small improvement (hopefully).
Now, no offence intended, but 3 of the 4 brands you've purchased are considered to be cheap & pretty mediocre in the scheme of things, chucking more money at them isn't gonna drastically improve quality or value.
Essentially, if they work OK, use them until they don't, that could be a long time potentially, & when they finally do break, sell them for parts & get the kit you really need.
As for the external cosmetic, your only limits are your imagination (& wallet), do your research as to what you need, which is not always what you want lol, it's easy to waste money on unnecessary dead weight that looks pretty lol.
You don't need to send them to anyone, plenty info & guides on the net, diy?
Edit: man above has faster fingers than me, but we're both singing from the same hymnsheet.
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