How do these games hold up?How's the replayability?How many hours could you get?Do these games work on W10?And which should I buy?

If you want to try a classic X-Com game you want to look at X-Com: UFO Defense. Its sequel Terror from the Deep is only really worth looking into if you already know the first, and the third game X-Com Apocalypse is very different and not as universally well liked (As well as lacking a finished source port so far).

The classic and the "new" X-Com game are very different, so it's not easy to tell if you will like one based on liking the other. Gameplay is very different in the classic games, with much more freedom of choice, but it's also less guided and probably harder to get into. But, it's really a great, unique game, and can be played very easily using the source port Open X-Com. I recommend looking up some videos to see if you'll like it or just giving it a go!

Far greater depth to the old games but it comes with a 25 year old UI. The pacing will be very slow and some times annoying if your not expecting it.
Works fine on w10 in dosbox for me.

1. Compatibility:
The games work fine on Win 10, you can use either the dosbox versions or the OpenXCom project.

2. Replayability:
Technically infinite, all missions are procedurally generated. There is a lot of terrain variability so it's rare to see two missions which have similar maps. That being said, after you play for a bit more (I have misspent literally hundreds of hours on UFO Defense and Terror from the Deep from my teenage years back in the 90s), you will start seeing patterns in parts of the maps.

3. Which should you buy:
Definitely start with UFO Defense. In terms of UI/UX there is no difference between UD and TFTD, but UD is more forgiving and quicker to complete - human technologies in UD are more powerful compared to the ones in TFTD (Laser >>> Gauss), and powerful alien technologies are a lot easier to obtain and research. Lengthwise, I could finish an UD campaign in a day or two no Superhuman, for a normal TFTD game it takes weeks.

Edit:
If you want an experience very close to the original X-Com games, but with more palatable graphics and UI, definitely check the game Xenonauts.

They cheap and there can be discounts so its not problem i think

X-Com: UFO Defense and Terror from the Deep the same game basically

but in Terror from the Deep and X-Com Apocalypse you fight against aliens from Lovecraft lore/world

and X-Com Apocalypse is really different and have realtime mode and have own issues baeacose
due to interrupted development and forced release ...

Also tehre is Open Source version of this games that have tonn of mods