Hi Phil.
I have just been through a similar experience myself so maybe my experiences may help you. My experiences relate to the refurbishment of LBSC's 'Betty'.
To aid the rebuild I purchased the plan set, so that I would know how the engine internals were constructed. Reasonable logic, but the original builder had his own design of regulator. I didn't realise this until I had managed to damage it. One of the main cylinders had a different size bore (much more than a couple of thou.), that may have been a machining error, or more likely to remove a 'blow hole' in the casting. There were other items that had that 'individual touch', but these were obvious before being dismantled.
So to sum up, my advice would be to buy the plans, but be VERY aware, that whatever the plans may say the actual build could be VERY different. As you said earlier, label everything and take plenty of photos and go slow – so you can think through the 'problems'.
You will have both frustration and fun, but you will learn so much. But the main achievement will be you will have a loco to be proud of and to remember your uncle by.
Best of luck. Alan.