Hi there James,
Regarding your first question of digital downloads in regard to disc versions; you're absolutely not missing out on anything at all, updates are fully supported on both versions, and some would say that the digital version is probably a better option to go for since discs are becoming more and more outdated these days, and it's still a question if X-Plane 11 will receive a disc version at all.
In regard to your second question, it is a much better idea investment wise to wait until you new computer is built, if you purchase one right now on your current computer, you won't be able to use it on your new one, so it's just a question of how long until your new computer is built and if you're willing to pay the full price two times.
I hope I was able to help.
-OscarX
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