As it should arrive in about a week. I thought I'd do a little diary to capture the inevitable trials and tribulations of migrating from one machine to the other.
Hopefully there may be some useful nuggets in here for anyone else contemplating the same adventure.
The story so far
Hardware
I'm not a computer expert, but I am a heavy user with a technical background. But when I started looking around I realised that what little technical knowledge I had was completely out of date and all the current specs meant nothing to me.
Based on the recommendations on previous threads here, and their presence on this forum, I dropped a note to Scan with my thoughts and a few queries. As well as my PC getting long in the tooth my interface is getting on a bit too and I was a bit worried about any potential issues there.
After an email chat with Pete I had a recommendation which I was able to run by Focusrite to check that things should be compatible.
There was then a bit of back and forth because computers have got bigger since I last bought one and the cabinet I built to house mine won't fit the standard 3XS case that Scan use.
But that's been resolved and my order placed.
So that's the hardware.
Software
I started noting down all programs and plugins I use to make a list of what I need to do to transfer them, where licence files are stored, etc. etc.
It turns out I have a shed-load of freebie plugins that I really don't need and this is quite an opportunity to cut back!
So I've noted down all the stuff I do use and started trawling website help pages to see what the transfer processes are. Where I couldn't find the info I emailed the developer to ask.
Particular Kudos to Diginoiz, Spitfire and Valhalla for their very quick responses.

I'm still waiting to hear from a few places though - if it goes beyond a fortnight I shall publish a list of shame.
Ancilliaries
Other than a couple of new USB cables I haven't ordered anything extra but I do have two screens, keyboards and mice so I will be running both machines together whilst I do the transfer.
Think that's about it for now, I guess the next update will be when the new PC arrives, but shout up with any obvious questions and I'll try and make a point of answering it when I hit the answers.