Although never done a tyre machine I have over 40 years experience upgrading machinery of many types, the best thing you can do is watch the machines in operation, troll through the drawings & get very familiar with the whole system, Talk to the experienced operators their knowledge is key to how it all works, try to find out if there are any existing problems be it mechanical or electrical there is no point upgrading a machine if it's not going to work as it did in the first place, try to get the company to invest in a total refurb, even if that is after the electrical upgrade. Remember, it will probably need the safety system upgraded to meet your countries current regulations this is important as you will also be personally responsible.
One final thing, Think about improvements for example making it easier for the operator, diagnostics perhaps an HMI even if it does not already have one, this can aid operators & maintenance staff by giving current status & alarms.
Not really required, but choose a control system that could also be integrated into some sort of reporting system, management like those sort of things (with the exception of the extra cost), it could be a feather in your cap if it is agreed.