Mass-effect – Are any of the Mass Effect 1 DLCs relevant to the story in Mass Effect 2

It is possible but inadvisable to play Bring Down the Sky from the very beginning. While certainly doable (I've done it myself) you're probably going to end up reloading a dozen times or more over the course of the mission.

The Batarians are no pushoevers, and you've got the always "lovable" varren to deal with, who have a nasty tendency to sneak up behind you and eat you through shields. There is also the issue of one of the torch bunkers spawning rocket drones nigh-continuosly until you take out all the batarian engineers - and everyone knows what sorts of naughty things close-quarter rockets due to Shepard, shielded or not.

I would recommend a level of about 25-30 -- the goal is that you have enough abilities by that point to survive a little longer than you might otherwise. But, level alone isn't the best indicator of difficulty in this game; if you've already beaten the game once, you should have the VII spectre master weapons available as soon as you can afford them, and a good gun and good knowledge of cover will serve you better than the extra 100 health successive levels will give you.

In Mass Effect, Sovereign, the Reaper that remained resident in the galaxy to montior biological life, was to have sent a signal to the Keepers on the Citadel to activate the Citadel relay and bring the rest of the Reapers back from beyond the edge of the galaxy. The Protheans modified the Keepers to ignore this signal, thus requiring Sovereign to have to assault the Citadel and dock with it in order to manually activate the relay. Shepard destroys Sovereign and buys some time for the Galaxy. My interpretation of these events was if Sovereign had been successful in activating the Citadel relay then the Reapers would have entered the galaxy at that point in time through that relay, destroying the seat of government and mopping up the galaxy from that point in space outwards.

Fast Forward to the very end of Mass Effect 2. A fleet of Reapers is seen heading towards the galaxy, under their own power. Rewind to Arrival; it is said that the Reapers will use the Alpha relay to spread throughout the galaxy. Doctor Kensen (was that her name?) mentions that with the destruction of the Alpha relay it would take the Reapers some months to couple years to reach the next relay. Since we know that all relays are interconnected it does not matter which relay they use really, but consider for a moment the location of the Alpha relay on the galaxy map. It is at the extreme end of one of the spiral arms, at the very edge of the galaxy. It is probably the relay closest to the Reapers who are moving towards the galaxy from outside.

To summarize: Protheans change Keepers and Sovereign fails to activate the Citadel relay; Shepard buys a little over two years time at the end of Mass Effect. Reapers "fly" to the galaxy from the outside (as seen at the end of Mass Effect 2). All mass effect relays are interconnected, so Reapers fly to nearest standard relay (such as the Alpha relay). Shepard destroys the Alpha relay, forcing the Reapers to fly to the next nearest relay. Shepard buys a little more time (a few months to a couple years) for the galaxy to prepare.