A actual HK will give you a MUCH better quality reciever and total rifle production/assembly quality than a CAI Cetme or their G3 clone.
True, but you get to the point of diminishing returns long before you reach the $2000-plus price point of a genuine HK91. THe CETME will set you back less than $400, and the quality of the recent CAI CETMEs seems to have improved greatly. For four bills, they are most definitely "plenty good enough".
The mag wells on the CETMEs will accept G3 mags just fine, and I've heard that they tend to function better with G3 mags than with the original CETME mags, some of which seem to cause issues occasionally.
Scope mounting can be done on both CETME and HK91 via claw mount, available at Tapco for $90-ish. They also sell a STANAG-to-Weaver adapter that lets you mount Weaver-base optics onto the STANAG claw mount.
Magazines are dirt cheap, as well...I just ordered another ten German surplus G3 mags from Tapco for $25 total.
Here's what a recent CETME looks like:
The rifle as it comes out of the box:
Right side magazine well, ejection port, and manufacturer markings:
Trigger group:
Chamber and barrel trunnion area:
Magazine well, bolt, and bolt head:
Rear sight paddle assembly:
Front sight assembly and muzzle brake: