Outliving your kids has to be awful. Can only wish you the best in getting over your loss.
Investing in a PC to last as long as possible is a low-value strategy. There's nothing wrong with it but it's an expensive way of doing things. Assuming the money is not a big concern for you, then your initial build, perhaps with the bump to a 6850K and 2x16GB RAM is a sound high-end configuration that should last a good while. That'll raise the cost to somewhere around £2600 in total.
If you're after a better return on your investment, a more mainstream build with similar performance in today's applications is going to set you back ~£1500
I'm completely guessing at lifespans here, but by the time the £1500 PC starts to struggle and feel slow, the £2600 PC won't be much better off. If you'd saved that £1100 difference, you could spend that on refreshing the £1500 PC with future hardware that will likely run rings around the £2600 PC. Here's an example:
Ten years ago, the state-of-the-art, £500 graphics cards of 2007 had 0.5GB of RAM and 48 "cores".
If you'd refreshed that with a £200 graphics card after five years, The 2012 replacement card would have 1280 "cores", each core more powerful per MHz than it's predecessor and each core running almost at twice the clock speed too.
Perhaps something like a 20x increase in real-world performance over just a 5-year span! So there's no easy way to future proof against that sort of progress, other than to spend less initially and keep the money aside so that when something big comes along and needs more performance than you currently have, you can replace older hardware as required.
Thanks Crispy. I didn't realise the X99 was getting on in age like me. That's all I've done over the weekend is research. I think I will wait for another month at least.