I need to keep access to the existing files for work, I’ve retired now so it will only be occasional, but nevertheless necessary.
Very good point.
Most importantly, do you have backup copies of your files somewhere else (e.g. on an external hard drive or another machine)? I'm asking because you don't want to lose anything in case the hard drive in the G5 dies. If something else in the G5 dies you can connect the hard drive to another machine and rescue the data... but if the hard drive dies, things get nasty.
My bright idea was to upgrade to a MP and be able to run my old apps on it in Tiger, but if this won’t work then I need a drastic rethink. I am concerned that the G5 will die at some point, either physically or technically.
There's a few options I can think of:
1. Buy another
PowerPC Mac and clone everything (i.e. Tiger, apps, files) from the G5's hard drive onto it.
A Mac mini G4, iBook G4 or PowerBook G4 can be had cheaply (from e.g. eBay), doesn't require lots of space and will run your software just like the G5, albeit more slowly. That way, you have a spare machine with an exact copy of what's on your G5, ready to be used if the G5 decides to call it quits.
2. Buy an early
Intel Mac, such as a Mac Pro that can run Tiger or Snow Leopard.
Adobe CS and other PowerPC-native software will run on there thanks to something called "Rosetta". So you'll still be able to access your files. The thing is, you probably need to reinstall Adobe CS and the other software on the Mac Pro (you can try just copying them over from the G5, but some apps may need to be properly reinstalled to work). Do you have the discs necessary to reinstall your software?
3. Do you have access to a more modern machine, either a Mac or a PC? If so, you could try installing a more modern version of your software (a trial version would be sufficient) on that and check if it can open your old files. (If so, you might want to convert the files to a format [PDF?] which can preserve the content but be opened without requiring the software used to create the original files.) But please try this with a copy of your files to ensure the originals are left untouched.
4. Get everything that is on your G5 (Tiger, apps, files) onto a more modern machine and try to run it as-is in an emulator such as PearPC or QEMU.
This will allow you to conveniently access everything you had on your G5 in a window on a modern machine, but it will be very slow and isn't easy to set up.
5. Virtualise an Intel copy of Tiger or Snow Leopard on a more modern machine.
This will be much faster than option 4, but isn't quite straightforward to set up either. You'd also need to get your software and files into the virtualised environment before you can work with them.