100% cromoly, possible hi-tensile steel rear end, you should be able to find some specs online. I have a soft spot for MBK's as my first proper mountain bike was a MBK back in 1989.
However, they are not great bikes and you will do better looking for other brands such as Specialized, Kona, Trek, Marin etc. If you want it because you like it and it reminds you of a past time when you were young, then go for it! However with the other big brands (there are loads more!) they will be lighter and ride better. MBK were always a bit behind the times.
Price wise? £50? Not a super desirable bike IMHO and with STX so fairly low end.