12s is a pair of derailleurs, some cables and battery if you want to upgrade the 11s at some point (wait until you're replacing cassette and chain anyway).
Tri setups for 12s use the same shifters as 11s, with a reducing junction to the new, smaller cables.
If the bike is a good price, you're not harming your future compatibility

Depends on your gearing needs. I do not like any of the new 12S groupsets because they solve for a problem I do not have. I use a 11-25 for most races and a 11-28 for hilly races. The smallest 12S cassette is huge and the cog spacing is worse than my range on 11S.

I'm just now running to the end of my 10 speed parts and moving to the new 11s stuff I've had sitting in my 'to be installed' bin for the last few years!

If my experience of using 'outdated' groups is any indication, you'll be fine for availability (and parts will normally be discounted).

I would say 12s is really only an improvement if you are going 1x with a 54, or greater, front ring but still riding some hillier terrain. The larger cassette sizes would be nice. The rest of the differences are pretty minor.

Maybe the best way to think about it is ask yourself how much you would be saving over the equivalent 12s bike. I bet that extra cog is setting you back 3-4000$.

At almost half the price of what these are going for on theproscloset you may even be able to ride it for a bit and sell it and still come out even.

As others said, I’d have a cassette or two on hand to tailor gearing for the course.

Of course, fit is a huge aspect of a bike purchase - so I’m assuming this bike is a great fit. (?)