If I buy halo reach from Microsoft store I wouldn’t need the master chief collection right?

It's an add on for the Master Chief Collection, so you would need the base game.

The Original Xbox version of Halo CE is only backwards compatible on the the 360
To play it on the One X you would need Halo The Master Chief Collection which includes
Halo CE Anniversary

Halo 2 Anniversary

Halo 3

Halo 3 ODST - available as add on content

Halo Reach - available as add on content

Halo 4

And at the moment it is HALF PRICE in the Microsoft store (with ODST and Reach included too)

Buy Halo: The Master Chief Collection - Microsoft Store en-GB

Halo 3 is part of Halo: The Master Chief Collection which consists of Halo 1 - 4.

https://www.microsoft.com/store/p/Halo-The-Master-Chief-Collection-Digital/BRNVR30J0SJ2

Halo Reach is not on the Master Chief Collection disc, only the Halo CE, 2, 3 & 4 are. If you want to play Halo reach, then you can purchase the dlc for the Master Chief Collection or download the Backward Compatible version.

Halo The Master Chief Collection. Because Halo 4 is there and in the future will have Halo Reach.