1. Most likely.
2. Assuming you bought it at a shop, they wouldn't sell you something illegal. In any case, there are aftermarket 18" barrels.
3. The muzzle flash, blast and felt recoil from a 3" Magnum will be astounding.
4. Yep. An aftermarket barrel is worth more than $175. Looks in pretty good condition too. You may want to put a bit of BLO on the stock. Oil finish stocks are your friend. Any wee scratch goes away with a bit more oil. Put a light cost of oil on the metal too.
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