Mtr comes with a very decent stock actually; other than lack of arca rail and adjustable cheek weld; it is pretty handy for hunting, hiking around with etc. I can see the value in getting into a chassis for adjustable length of pull, cheek weld, ability to add weights for bench shooting and installing an arca rail etc.
if you are planning to go down that road and want to spend money; look into Spuhr chassis. AI at-x chassis is also very good but not mlok either. MDT, MPA, KRG, Vision, XLT are some famous flavors offer something for most tastes. Best of luck