All goodie upgrades that I usually go to is here
Often Free and relatively fast shipping: Ehobby Asia
No you cannot order guns from there they get stopped by CBSA and you lose your hard earned money.
(Just a generic answer for anyone else reading)
As for which gun is easily upgradable?
Its all personal preference but I like
Version 6 Gearboxes they are easy to work with and not too hard to re-assemble but some may find others easier to work with.
So it shouldn't be a matter of how easy it is to upgrade and put parts in but a matter of how available are the parts for the gun that makes upgrading easier since you will have a vast amount of parts in case you screw up
is local in Canada and you can get many goodies there too and you are supporting a Canadian retailer
What I have found is that there are many AK parts available which are Version 3 gearbox parts are all over the place.
But if you want to go simple like me, most of the time I just get a Modify 90 Spring which puts me at 330 FPS or lower which allows me to play at any venue in Ontario
Anything around Modify 100 and up type of spring requires metal bushings because the pressure may be too strong for normal plastic bushings that are usually found in stock gearboxes.
But definitely field those guns first, play with them, rent them somewhere to try it out. Its best to get a gun not because its l337 but a gun that feels good in your hands and suits your style of play.