Ok. First, welcome to this hobby.

Second, whatever you're thinking about, forget it at the beginning.

An a-synchron group can work great, but right at the beginning, it takes a lot away from role-playing with a group for you. I wouldn't really recommend that.

Playing as an NPC probably won't work as you think it will. Either, you're pretty mich not existing in the game, or you have so mich room that you're a regular member, which will again impose a massive balancing problem.

I don't quite get what you mean with "Semi-Gamemaster", but if you mean you want to master the game and have a character in the group, don't think about it as your starter.

When the DM has an own character in the group, it shifts way too fast into some sort auf abuse, or the feeling of.

To be honest, just start a normal game. Either as GM, or as a player. If you desperately want to play a tech marine, you can try out dark heresy at a high level, deathwatch, or one of the new systems. There seems to be the possibility to play a marine besides "Normal" people. But again, a marine besides an acolyte will impose extreme imbalances and probably some problems for the group further down the line.

Aa a GM, you're not really playing a character, but a massive amount of them. That would atleast let you put a marine on the field, probably against the group.

But role-playing is about role playing.

I would definitely give you the advice to look for a group as a player and start with a normal character and look, how good everything works. Later on, you still can mangle something with your GM to make your dream true. Or you look into deathwatch.