Cyma are brilliant, depending on what you want. I can recommend two from personal experience:
Cyma platinum SR-25 as your more expensive option. Comes in around £330, but does everything amazingly out of the box and the internals are really, really nice. I took mine apart and was thoroughly impressed with what is put into it! The MOSFET isn't amazing and is just there to protect the trigger and apparently also protect the gearbox from locking up on semi which is nice, however it's not programmable at all, so making it semi auto means you'd need to modify the selector plate or something.
Cyma M14 as your cheaper option. Comes in around £150 and will shoot pretty damn well out of the box. V7s are pretty easy to work on once you get to the gearbox, but they're more time consuming as getting the m14 apart takes a bit of effort. All I've done to mine so far is swapped out the hop, nub and barrel (so about £40 extra? ZCI barrels are great) and it's firing at 1J and hitting out to decent range. Probably won't DMR mine, but I do plan on doing some more work to it once my SR-25 is sorted and I can take the lessons I've learned there to my m14. If you did want to DMR one, you'd probably need to swap out the piston for a metal-toothed one at least, but the other internals I imagine would hold up.
CYMA are a good shout as they have robust internals. The M14 is cheap, but harder to work on than V2 gearbox guns. I'm not sure on how you'd modify the fire selector on those.
I accidentally modified mine (I snapped it when I was working on it by accident ?) and found out it's suuuuuuuuuper reversible, but not so reversible that you wouldn't be allowed to use it as a DMR. When you separate the receiver from the stock there's an arm that goes around the top and moves in and out depending on semi or full auto. All you do is remove a screw from this arm and detach one side from the other and you're immediately fixed to semi auto.
Pistols, meh, I run an MP5K on a leg panel as a sniper/DMR secondary. Full size V3 gearbox; full auto; light, cheap mags including a 600 round high cap if you want to just slot one in and forget it. On cost, rate of fire, and weight-per-shot, it beats a pistol hands down. Other opinions are available though.
From a purely practical perspective, I will second this (though I prefer pistols because a GBB is fun to shoot and I just like pistols a lot). I also have an mp5k and I can say they're fantastic little guns, though I run mine on a sling. Again, Cyma, £150. All I've changed is the hop rubber and nub again and it's sending .32s to 60m. As
@Rogerborg said, it's full auto, full size gearbox and it's just all around great. I run 65rd stubby midcaps in mine, but the 600 round double high cap also works. If you want the best performing secondary alongside a rifle with an MED, this is your best bet, and will probably be a cheaper option because the mags are dirt cheap, unlike gas pistol mags that are fairly pricey.
For a more expensive option, I love the TM mp7. Same thinking, but it's an AEP (or gas, but I don't like the gas one) so it only shoots about 0.6J, however I've got mine spraying full auto to 45m fairly accurately which is all you really need alongside a DMR and it's slightly easier to carry than the mp5k because the mag goes into the pistol grip.
Surprised no one's mentioned an AAP-01 as a sidearm, it matches the OP's criteria...
# Good internals/externals: Over all yes, however regular full auto will shorted the hammers lifespan.
# overall good pew: Yes
# good fire rate: Yes
# not too expensive: The price is great for such a good pistol
... and I run mine as a sidearm with a CTM holster and can't fault it at all (using 0.28g with an ML Autobot 60 degree).
In terms of pistol, this is a great option and it's cheap! £90 or so and it has an absolute boatload of aftermarket parts, including drum mags and carbine kits. I've converted mine to a Ruger .22 to use at Vietnam games because it's so good and reliable I wanted it to replace my 1911 and its tiny magazines! So much more reliable, which I need as a sniper; a good sidearm is just as important as a good main rifle in my opinion!
Outside of that, just get whatever TM pistol you like. They're pretty much all good (though I've heard the old m9s are a bit crap), though you need to watch the 1911s because the mags are so small they don't hold much gas. They still shoot well, but outside of summer they can struggle to cycle through a mag on one fill of gas. I've owned about half a dozen TM pistols and not one has disappointed outside of the 1911 for the reason I said above.
Overall, I wouldn't get hung up on full-auto for a pistol if it's gas. Whenever you fire, there's a cooldown effect each time and gas doesn't like being cold (it just doesn't work as well when cold). When you fire a gas gun on full auto, it loses all consistency and sprays BBs and gas all over the place because that cooldown stacks after every shot. You also don't need full auto though; I've been running BASRs lately at my local site, even though there's a lot of close range areas where it's useless with its 30m MED and I haven't once gone away from a firefight thinking "if only I had full auto...". Snappy GBB pistol has been all I've really needed, and my various TM pistols + AAP-01 deliver that in spades.
