Should i buy a MTB helmet

Not unless you intend serious gnarly stuff.

I can see why cooldad made that recommendation, fully understand it in fact. But now is the time you need more protection. You are about to start riding off-road, so instead of nice smooth and flat tarmac, you will be faced with rocks, ruts, mud, overhanging trees, and roots - don't forget the roots! Also very steep inclines, off camber slopes, loose terrain, and so on. You will have suspension that needs to be set up to assist you instead of generating problems for you.
Intellectually you know about all this, but it is not embedded in your muscle memory, and your fast twitch responses are not programmed. You are unprepared for this.

I still have regular offs and I have broken every single helmet I have ever bought except the two I have now. My scars are multiple and I have stiff shoulders from the crashes. I only call it a crash if my feet go above my head, otherwise it is an involuntary dismount (but even they can still hurt).

The two best things I ever bought after a trail helmet were elbow guards and knee/shin guards. I should have done it years ago when I was a newbie, not later on when I was more experienced. I sure would have fewer scars and have spent less time off the bike hobbling about. I have spent more money on physiotherapy then ever I did on personal safety equipment because I never spent the money on the pse when I should have done.

So despite having the money, why didn't I spend it on pse? Simple answer: I was embarrassed! Yes, embarrassed. Embarrassed to be in kit that I only ever saw "experienced" and "better" riders than me wearing. I thought it was almost a badge of office that I was too much of a newbie to wear. To my mind at that time, it was building up expectations that here was a rider who knew his stuff; expect to be astonished! When in fact I was falling off the bike at every opportunity. Whoever coined the phrase "all the gear and no idea" has a lot to answer for. I was stupid. :oops:

Get yourself some decent kit, you will get value from it. Ignore anyone that says otherwise and have the self confidence to use it.

Oh, and go on a few mtb skills courses. Another high value for money thing to do. It will also maximise your subsequent fun.