What you actually do is, of course, up to you.
If I were you (and I am looking at a similar job), I would aim to upgrade the lot. The system I would choose would be:
- S7 PLC
- Citect V6
- windows XP
- Comms via Ethernet (which I don't think you could do on the S5).
Some important points during any upgrade, are:
- How much down time can you get?
- How to ensure the system won't lose any functionality
- What steps you need to take. For example, upgrading from Citect 4.2 to 6 would require several steps, such as upgrading to 5.0, then 5.4, then 6.