Bro, since you plan on using the car as DD, the STI might be a more practical option. I, of course love the Z more but for functionality and storage, the STI is the way to go. You've done well
ringland issues are rare on the newer models. 2008 was a really bad batch. i've tracked my car, i've seen wrxes and stis on the track far longer than i have, and they've had no problems. it's not perfect and it's not a guarantee, but the ringland issue has definitely died down in latter model years.
No you use two buckets, each with a grit guard at the bottom. The grit guard keeps the water below it from swirling so all of the dirt settles at the bottom and doesn't stir back up into the water up top. You use the clean water bucket to rinse your mitt out in before putting it in the soap bucket.