Your stuff, your decision, but I think remarking a HK receiver is like adding Rolex "Presidential" marking on a plain Jane Rolex. Even with a sear in the SP5K it will have full auto function, but it will never be a factory registered receiver machinegun. Most people will not know the markings, those that do, and are fooled, don't know the difference. Those that do know the difference, won't really care. If the markings make you happy, good for you.
The last I knew the new style receiver block needs to have the legacy shelf or the modified bolt on trip. Those were the only approved configurations. The trip seems like it would be a PITA to unbolt the trip every time you wish to remove the bolt group. Maybe things have changed. But I would think that the legacy shelf allows the use of a standard carrier.
Congratulations on two sear purchase. Good luck with the conversion process.
Scott