I am 5’9″ and mines 18 inch.
If you go to halfords they shoild have one built in either 18 or 20.
And you can have a test ride to see if that is the wrong or right size for you .
I keep my chain well mantained and it shifts no Better or worse than the xt set up I have on another bike.
The brakes stop very well, on my bike with xt brakes I can stop very quickly without locking them up where as the tektro brakes on the hoodoo do lock up easier if you are to heavy handed but on the whole are very good.
As I said earlier I was thinking of swapping out the crank for slx or xt but I think the only benefit would be shedding a little bit of weight as everything works great so I don’t see it being worth the expense until it wears out.
The only things I have changed on mine is the front tyre for a bit more grip, seat for flexier rails as I have a temperamental back and pedals as the originals one started clicking after 6 months or so as I smashed it up
a bit.
I have read some people saying the thought that the coils where a better option but I could not say as I have never tried them.