Sounds like you've done a great job doing your research so far, bravo. To answer your questions, we'll have to unpack a little bit, as they're hard to answer because they're not exactly the right questions... again, totally commendable, but in the effort of best answering them, here you are...
As far as the modern "moonwatch" goes (by moonwatch, we mean a speedmaster professional with a hesalite crystal and solid case back), is has seen very little change since the moon landings in 1969. So for our discussion, they're all "originals"... In short, the only notable difference you will see in a moonwatch from say, 1997, all the way to one that you buy brand new in a store today, is that around 1997 they changed the dial from using tritium - a lightly radioactive substance used to make the hour markings and hands glow in the dark - to using a substance called "luminova". You will be able to tell this differerence in any speedmaster you see by looking at the bottom of the dial, just under there 6 o'clock marker where it says "swiss made". On a tritium model, it will say "T swiss made T" (T for tritium). On a luminova model, it will omit the T's... Over time, tritium is known to decay and usually turn to a creamy yellow color, and eventually will stop glowing. Luminova doesn't do this, and will stay bright white over time, and will retain it's glowing properties. Functionally luminova is better, but collectors like tritium because it ages and shows an attractive (in the opinion of many) age over time. We around here would generally not call any model made after the 1970's "vintage"... but if you were to take the term much more lightly, I guess you could call the tritium/luminova as some sort of line of demarcation. Perhaps you could say a tritium model might "hold it's value" better as you ask, given that it has a slightly greater commonality with true vintage speedmasters, but it's really just speculation and there's probably not any reason believe that or to let that be a deciding factor.
So, if you're trying to decide on a model for your partner, every model past 1997 is essentially identical. From there you can simply decide what condition you want the watch to be in, do you want it to have all of its original box and papers, do you want it to have it's manufactures warranty, etc... If you're looking at something older than that, then the rabbit hole goes deeper and we can totally have that conversation. But, it sounds like you and your budget are honed in on a modern speedy. Cheers. Let us know if you have more questions.
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