Can he handle a 97K? It's quite heavy and with a long length of pull. I imagine in a couple of of years no problem but 12 I'm not so sure.
1.Mk.1 or mk.2 doesn't matter, all internals are the same. The difference is the barrel length and muzzle weight/end catch plus minor stock shape changes. Mk.2 looks much nicer IMHO. this will be important to someone his age.
2. About 2/3 new price is about right.
3. I wouldn't pay any more for tuning kit fitted because you can't tell what's in it unless you strip it. I'd only pay extra for tuning if I knew the seller or if it had been through the hands of a respected tuner and had paperwork.
4. Most things are fixable apart from severe pitting inside the bore in which case it'll spray pellets all over and most likely will show signs of being abused or neglected. But obviously any issues can cost money to sort so price should reflect this. Well I think they can be re barrelled but that's a big job for a skilled gunsmith or machinist.
If at all possible, shoot it on a range at actual targets. That plus the external condition will tell you all you need to know. If anything rattles or grinds on cocking, that could be a broken spring or something else. If it cocks and shoots nice and smooth, doesn't kick like a mule and groups well, then it's probably ok. If it's otherwise well mannered but twangs that's easily sorted.