No reason they can't be immortal (a robot probe or self-repairing machine intelligence kind of thing
or you could go with the andromeda strain approach. "don't send a signal or a machine or a person, rather send the coded information to build the machine/person to deliver the message".
yeah, there you go. they didn't come from andromeda, they were seeded from andromeda, and are constructed complete with memories and skills.
heh. think of the clone factory in star wars. yeah, that's the ticket, they conquer galaxies, they're here to conquer ours. a civilization with an intergalactic/intermillenial frame of reference to reality. why conquer? they don't know, they're just programmed that way. perhaps an enterprising traveller character with computer 5 and medical 5 could reprogram them ....
hey, perhaps it's not THEIR civilization, they're just the military slaves, perhaps the real aliens are on the way in some long-term sleeper ship and are due to arrive at the prepped invasion zone soon. crystalline life forms, yeah, that's the ticket, they live millions of years, are highly individualist and require entire planetary systems for each individual which is why they need entire galaxies, breed (share crystal code) only intermittently, slowly spreading through the intergalactic universe ....