Those will work fine. You can get them cheaper from another vender. Shimano Tiagra 4603 165mm Triple 30/39/50t Crankset Silver~ Bottom Bracket Not Included - AEBike.com

as tsl showed with the links, you don't really want to "use just the arms." But, you can save the rings from your original and use them down the road if/when the rings on the new crank wear out.

You already have a bottom bracket, so fortunately that's not an issue... just reuse the cups/bearings that are currently on there.

Quality is the same, both are using stamped rings (as opposed to a forged ring) which is true all the way up through 105 (105 rings used to be higher quality). Honestly, I don't see any reason to justify buying 105 over Sora, let alone Tiagra. Bearings are the same regardless unless you want to buy something aftermarket (independent of the crank itself), so bearing quality isn't an issue. If there's any difference besides that, it would be in the weight of the crank arm; I wouldn't worry about it. I don't think the crank arms get the hollow treatment until Ultegra anyways, but I could be wrong (hard to keep track from one generation to another without running off and researching it).