As has been mentioned, the biggest problem with the SAR-3 FCG is the inconsistent effort made by the importer to modify their standard cast FCG to fit the SAR-3.
Since you are buying through a shop, you should be able to evaluate the FCG prior to purchase. As was noted, what you want to avoid is a rifle where the hammer face is incorrect and does not strike the firing pin squarely when rotated farther forward for the SAR-3. If the pin is struck squarely by the hammer, then you may well have trigger slap and the trigger pull will not be wonderful, but the gun should at least be reliable.
Alternatively, you could grind the hammer to strike the pin correctly, and replace it down the road after it mushrooms out, and you want a better trigger anyway.
The G-Tech parts would require modification of the hammer as described above.
Another choice would be to purchase our hammer alone, which would solve the geometry problems, then possibly buy the trigger/disconnector later to achieve a better pull.
Good luck, Charles