ECU

[Bline]

Hi all, I am a bit confused, I recently bought a Calais that has been sitting for 18 months, put a battery in and nothing except fuel gauge error Ok ecu is dead , can start it by paper clip in relay no probs and keeps starting for a while afterwards then nothing after leaving it overnight, so reading thru the posts here I gather I have two options, replace just the ecu and get it programmed? Or replace ecu, bcm and key etc
Do I have that right?
Do I need a Calais or similar or would any 190kw model do?
What do the different specs mean e.g Low, Mid. Lux, High?

Thanks in advance for any input and help