Goflux:
By the way, do you start with an Advanced Specialty at rank 1?
If your asking what I think you are it depends entirely on the advanced specialty. For example, in Rites of Battle the Deathwatch Champion has a perequisite of Rank 4, whereas the Deathwatch Black Shield has a perequisite Rank 1. The advanced specialties have a list of perequisites for the specialty.
That answer your question?
Goflux:
Question: What're the most important things to get as a Tech Marine?
I will try to answer this, but I have never played a techmarine before. I would think, probably obviously, repairing equipment - so repair skills. I would think something similar to an Adepetus Mechanicus priest, since Tachmarine is also a part of the AM. I would probably take some piloting skills as well.
Goflux:
Why does nobody post here?
Question: Are genestealers a suitable enemy for a new Rank 1 Death Watch group?
Personally, I think genestealers would be acceptable - possible as a first encounter with the tyranid. At least I would to players in a game.