Getting the barrel and chamber extension mounted right are a huge issue with the H&K/CETME and it is the number 1 issue with CIA guns. The unit MUST be pressed and aligned properly or you will have issues.
The gunplumber is a great choice for getting a G-3 or CETME built right and I believe that you would be far better off in the long run to have Mark do the heavy lifting on this.
As far as the H&K/CETME versus the FN or AR or M1A, It depends.
The M1A is a very good weapon, but maintenance requirements are higher and the weapon is more fragile (this is relative of course, a wooden handled hammer is less robust than a estwing but neither is likely to break under even heavy use).
The AR-10's are fine target rifles, but IMO, they have no place in a firearms battery designed to keep you alive. The AR has always had issues with dirty ammo, and we want to use it for 7.62 NATO? Say again? AR-10's have all of the disadvantages of the AR-15 but none of the advantages of light-weight, low-recoil and high-capacity.
The FN is a fine weapon, but again, it is a higher maintenance weapon and not quite as reliable. The Paratrooper version is actually more reliable due to the elimination of the rat-tail recoil spring system. The FN would be my second choice as a SHTF weapon.
The H&K/CETME is one of the most reliable and robust rifles around. Yes it does suffer from bad ergonomics, but this is more about the one-size-fits-an-AR training used in to many schools today. If you learn to run an H&K properly you can run it just about as fast as an FN which has far superior ergonomically. The delayed blow-back operating system is the most forgiving of beat-up dirty ammo of any weapon you will find, I have run a CETME with ammo I wouldn't think of putting in an FN. The rifle tends to be very accurate IF it is put together right, the bad ones tend to be CIA butcher jobs, but you will need a good trigger job to get the most accuracy out of the weapon and again Mark the gunplumber will give you a decent trigger. I don't know where the "inch/Metric" thing is coming from, that is an FN issue, not an H&K/CETME. Likewise ANY barrel for this weapon has a fluted chamber, some don't have good flutes and will not work as well as standard barrels, but they are all fluted. You do need to be aware that H&K barrels use polygonal rifling and are bad to use with un-jacketed ammo, not as big a problem with a rifle as a pistol but good to know. I would get a paddle mag-release and a trigger job from Mark or Bill Springfield
http://www.triggerwork.net/hk9xrifles.html and run with it.
hope this helps.