The action is an M18 action, it's not based on anything else as far as I know. As you noted, only the scope base screw holes were made to be compatible with Remington 700 LA base. That was more than likely to help keep costs down by not having to design and manufacture a proprietary scope base mounting system.
When it comes to aftermarket support, the M18 is pretty limited, Boyd's makes some laminated stocks for it but that may be about it. Oryx makes a chassis for the Mauser M18 but my understanding is that they only sell it direct to Mauser for their M18 chassis rifle. I'm not sure what brands are available in SA but a Howa may be a better option if you're looking to upgrade a rifle and keep costs down.