It depends on the 12 year old.
It's a big heavy rifle if you get a wooden stock, a plastic stock will be a little lighter but not a whole heck of a lot.
1) Not a huge amount of difference between the two.
2) £350 ish or less depending on how it's been monkeyed about with.
3) Tricky one; whilst tuning kits can radically alter the feel of the gun it does depend on what's been changed, and who by; generally don't pay more. That way you can have the fun of doing it yourself a year down the road.
4) Not really, they're built like tanks. Rusty piston/sleeve if they've been shot on a rainy day and not dried out properly.
If he's already shot one and liked it then go ahead, I love mine.