1) does it feel like ravenor/eisenhorn or more like guant's ghosts or the horus series?
It's slated as a game that is focused on investigation, so more Eisenhorn than Gaunt's Ghosts (i.e. combat focus). You can play it either way, of course, but you're going to have to spend a lot of time customising it yourself if you play away from the central themes of the game.
Oh, but one thing that you're going to have to remember is that you don't take on the role of the Inquisitor. They are a nebulous patron figure who hands out the missions and whom you might never meet. Rather, you play "acolytes" of the Inquisition.
2) how approachable is it with little to know RPG experience?
One of the strengths of Dark Heresy is that you can basically sit down, pull out the book, roll up some characters and start play. The simplicity of the system also means that whomever elects (or is forced! ) to be the GM also has a much easier time of it. The system is also evocative of the more "medieval" approach to the 40k setting, so in many ways it plays you into that mindset for the Imperium. If you were going to hit any stumbling blocks it would be that, arguably, there are some absences in the background to the game. Since you've played the table top wargame, though, this shouldn't be a problem, so all is good.
3) how essential is having a large group, or in other words is a group of 2 people going to get a lot out of the game?
Not essential at all. One player and one GM is all that is needed.