In the back of my mind they I know they all take STANAG mags but I was thinking moreso the exterior platform to worry about. Also I remember someone telling me about barrel length and accuracy and it was determined (from some quick back of the envelope calculations) that there were diminishing gains with longer barrel lengths and other factors come into play like stability of the gun itself because the BB isn't going that fast and jerking it after you pull the trigger could mess with the trajectory. If you're worried about carrying something unwieldly there is the option of a 10.5" ("CQB")M4 and if you want more length changing the barrel and flash hider to a longer barrel and covering it up with a suppressor.
Oh yeah, one more thing that I thought of which you wouldn't know otherwise but is something to consider.
If the gun takes "STANAG" mags it doesn't mean that it will accept all STANAG mags but doesn't mean that you can't modify it to accept it. For example the TM FAMAS which uses STANAG mags actually doesn't and you need to file down at the mag catch or the square where the mag catch sits and the front of the mag or inside of the magwell. The mod for the FAMAS is well documented
. I also wouldn't recommend a FAMAS because of the V1 gearbox design so your choice of the L85, Aug or Tar21 are fine. Parts for the V1 are pretty much impossible to find but the ICS and CA are solid, I guess they're calling the CA "X-Series" the Proline now? Just make sure you don't get the CA Sportline, I remember it being crap (and for the price point the Sportline tries to compete at you can get something better for the money from a different brand).