Hi Andreas,
From the information shared
You have ERP 6.0 EHP6 with 200GB size. It does not look for me a very old system for which decommissioning is required.
1) Does the customer need data to be reviewed once the system is decommissioned ?
2) What is the extact reason behind decomissioning ?
3) Please be aware that SAP ILM requires license which may be additional cost overhead.
If the system is not so critical, you may take 2 sets of offline backup and preserve it.
Additionally you may have a small size server where you can restore this backup and keep the system read-only for audit and legal purpose.
Hope this helps.
Regards,
Deepak Kori