As far as I have heard, pretty much all of Shimano's latest groups are pretty good. I have experience with the last-gen groups, and can confirm that they are decent, Tiagra good, and 105 (5800) is an amazing step up .... but Tiagra (4600) is quite good, and 4700 is supposed to be better.
As for Sora vs. Claris, definitely go with Sora. Everything is just a little more durable, the front derailleur flexes less, and an extra ratio will never hurt. If you plan to ride the bike for several years, the cost amortized over a couple thousand days in minuscule. if you ride "a ton" then ever bit of quality and longevity, however slight, will add to your enjoyment.
I agree with @
shelbyfv, buying used shifters is a risk. I have bought a few sets, and only got one bad unit ... but because shifters cannot really be rebuilt, you have no idea if you are buying a few months of light wear or few months of life left ... I have zero issue buying used parts otherwise, but I always look for "new--take-off" or parts with a lot of pictures showing all the wear marks. The down side is spending a couple hours every other day scanning EBay waiting for the magic deal to appear.
If you ride a lot, spend the cash. You will never regret money spent on the bike, but you might regret money not spent, in my experience.
For my tastes, just having the extra gear and slightly better longevity matters most. The couple hundred grams weight isn't going to make a difference in riding enjoyment. Sharp performance and ready ratios will. Speaking of ratios: Shimano offers 11-30, 12-25, 13-25, and 14-25 (for junior racing, I’d assume.) However, I imagine just about everyone else's cassettes work, including Shimano MTB. Shimano's 9-speed Alivio cassettes range from 11-28 to 11-32, 11-34, and 12-36---though you'd need the medium-cage Sora derailleur for that last one.