Should I buy Morrowind?

Yes, there is nothing else for me to say but just; Yes

if you want a decent challenging introduction to the series you can start from morrowind. if you want to be lazy and have cruisy gameplay start with oblivion.

Either Morrowind for Challenge if you like Dice Roll RPGs (Your swings do not connect unless a dice roll you cannot see says they do, it is basically a First Person Never Winter Nights), If you just want to explore and mod, Skyrim.

Oblivion is boring and is just a generic medieval fantasy that was rushed. Its lead Designer went on to make the Failure "Kingdoms of Amalur"

probably

If you enjoy RPGs, mods and can stand outdated graphics/animations. Good game

I think I better start with Oblivion cuz Morrowind looks way more hardcore and it doesnt look like the better game to start with the series (for me).

Playing Morrowind first for sure but you may hit a wall:

1) You won't like the graphics, animations and gameplay mechanics combat in particular and it feels a little clunky - basically it is an old game. Back then it belonged to the category of "This game is epic we will forget about the shortcomings". With time things like graphics and all the shortcomings even back then have just become more apparent. This game totally rides on a great fantasy world that you are allowed to explore and it is arguably the best of the entire series as it hit the sweetspot of world scale and ability to make every place unique and interesting. Oblivion feels like an everyday medieval rpg after Morrowind. Skyrim is between Oblivion and Morrowind in terms of how cool and unique the world is.

2) Now you might want to modernize your game a little. Depending on what you find lacking and if you can accept that you will be playing through the game not 100% original content wise you might get a total overhaul of the game or simple graphic updates.

3) If you play Oblivion before you will probably make it that much harder to adjust to an older engine. Always easier to start from the beginning when playing through a series as it's easier and more fun to see how the series advanced engine wise. Some people have played Oblivion first and played through Morrowind just fine though so it depends on the person.

But seriously Morrowind rode on just being an epic game. I played when it came out and even then I remember I found the combat system a little basic and animations quite horrible. I do still remember that at that time the graphics were amazing, well they actually were too it's not my imagination. They got praised. It helped that the art style was pretty effing good.

Oh and from a general story perspective you should probably play Morrowind first as it advances the timeline, although the events are not that intertwined you get references here and there that would be missed if not having played Morrowind. So all the games share the same world and allows you to enter Oblivion with knowledge of the world and better orientation of who is who and what faction,race etc relations are like.

So try Morrowind first and if you absolutely hate it go for Oblivion. At least try to get past the starting village in Morrowind and use mods to help modernize the game.

If you love RPGs you should not skip Morrowind in any case. Go play that shit right away!