What kind of MTBing are you planning on doing? If you were to buy a Shimano XT, even just for the fork, you will be spending more money on it than your bike is worth. I agree that spending some money on a better bike is the best idea. Even for 1,000 to 1,500 you can get a good quality hardtail. If you can’t put out that kind of money up front most LBS have same as cash, 12 months to pay, zero interest, so you can pay a bit at a time. Just my opinion.