I highly suggest you play them as soon as you can. As for DLC, the bigger ones are usually worth it, I forget names at this point, but for ME2 make sure if you have the ability to, grab the one that mentions Heists n stuff for a new character (plus the free ones obviously) and in ME3 grab the Citadel DLC. They are kind of pricey, but those (mostly citadel) are great additions to the game. Oh and the Free ending expansion they made (that I still think is good, fite me
I try not to pre-order out of principle, but those gameplay trailers for Andromeda they are making are pushing me close to wanting to. The balance of exploration and linear areas (basically just choosing what areas to go first really) seems like a good balance between big open world boring games and tight linear action packed games.