I know you're trying to help, and it is a good idea, but I have thought about this.
My Hardrock already needs new parts, which is one of the reasons why I'm going to buy a new bike. The chain, gears and dérailleurs are very worn. They still work ok but with the use I give them I'd have to replace them relatively soon. The wheel rims are also pretty worn (V brakes) as are the calliper pivots.
Were I to fit these parts to a new frame I'd have to replace them before long. Plus I don't want to buy a frame with 'V' brake lugs and be left with them when I fitted disks later. To avoid that I'd have to buy both disk brakes and disk-compatible wheels from the off. Those two items alone would take the cost up to nearly £600.
While it could be done...just, it would result in a heavily compromised 'new' bike and importantly a non-functional Hardrock. If I buy a whole brand new bike the Hardrock gets to live on, which is ideal as my eldest son is in need of a bigger bike. Giving it to him saves the cost of buying him a new one, and as he uses his bike so little the worn transmition will probably do him for ages.
I'm not taking chances on it though. I'm replacing the full braking system with new, getting the fork checked and serviced and checking the frame for cracks. The whole bike will be stripped bare and checked out, but it'll still cost a lot less than a new bike.
He's quite happy. He's tried the bike and can't believe how nippy it is. I'm quite looking forward to tidying the bike up for him, it'll be getting more attention than it got when I was using it ;0)