I would like to add to the speculation. If you do some Internet search, there is a project with a giant telescope known as the Colossus (interestingly, it is the name of one of those unofficial tracks released by Sam Hulick). This real life scientific project intends to find signs pointing to possible life or inhabitable planets in Andromeda. (my bad, I read this a long time ago, I meant far beyond our solar system and it would include the Andromeda in a fictional future where technology actually permitted this feat).
How does this tie to what we know about Mass Effect? The chances of finding Earth-like planets is quite limited, since there are many variables involved in the formation and evolution of planets: relative distance to stars, presence or lack of water, etc. So my gut feeling is that Ark initiative leaves the Milky Way with a degree of knowledge of how the Helius cluster includes "planets of interest" where, upon confirmation and exploration, life could be possible. The film "Interstellar" also presented a similar scenario, with a limited number of planets detected that may, possibly, harbor life and sustain human colonies. The trip to Andromeda is such a long voyage, that it is hard to beluieve they would not to some "research homework" before picking the helius cluster as destination.
I would also like to mention the presence of multiple krogan in the several videos we have watched (official trailers and leaks). As you know, the krogan's physiology allowed them to survive in very hostile environments with minimal life support. I am not limiting my statement to Tuchanka. If you read the codex and Avina's description of the Rachni wars, you will remember how the krogan were able to fight in some of the very hostile planets inhabited by the Rachni. It's not specified, but I would hypothesize that these hard conditions could include low oxygen levels, gravitiy force far superior to Earth, the existence of fauna and flora that could be very poisonous for Council races but not so much for the krogan's redundant systems etc.
The presence of krogan, regardless of the Ark's departure timeline, could be a necessity: you need scientists and very bright people in order to develop a civilization in a new galaxy, but you also need workers and combatants who can survive the possibly arduous conditions of many of the inhabitable planets you will find, and the very likely possibility of hostile presence in some of these planets.
For these reasons, the expedition will need to include a military or military-trained complement of settlers to protect their assets. including Alliance and other Council races' military personnel. Some of these soldiers, given the selection process, would be highly skilled soldiers (N7, Salarian STG, Asari commandos) The selection process of people travelling to Andromeda was probably subject to heavy scrutiny.In order to develop a sustainable civilization in a different galaxy, you would need some of the brightest biologists, geophysicists, engineers, etc.