Hey there! I totally get your situation - inheriting a 15-year-old mixed PLC system can feel overwhelming, especially when you're trying to balance budget with future-proofing. Here's what I'd suggest based on what I've found:

For the Siemens S7-300 systems, the smartest path is migrating to S7-1500 using Siemens' TIA Portal. They have built-in migration tools that can convert your S7-300 projects to the newer platform. This gives you modern features like better diagnostics, improved performance, and long-term vendor support. The key advantage here is that Siemens provides a structured migration path that preserves your existing program logic while upgrading the hardware.

For the Mitsubishi FX series, you have two main options: upgrade to the newer FX5U (iQ-F series) or consider moving to Mitsubishi's modular Q or iQ-R series if you need more scalability. The FX5U offers a good balance of compatibility with existing FX programs while providing modern features.

Here's a budget-friendly strategy: start with a phased approach. Prioritize the most critical or problematic PLCs first. Document everything thoroughly before you start - this is crucial when dealing with legacy systems. Consider keeping some existing hardware as spares during the transition to minimize downtime.

The smartest move is to standardize on one platform long-term, but given budget constraints, focus on getting both systems onto their respective modern platforms first. This gives you breathing room to plan a more comprehensive standardization later.

Would you like me to dig deeper into any specific aspect of this migration strategy?