That setup will serve you well. The spring rates technically do matter... but it's not something that I would worry about when on a budget. They do make automatic and manual JDM Pink springs (auto is heavier), so you could look for the auto pinks.
If anything, the weight of the H6 will lower the front more and counteract some of the saggy butt.
Basically, devote some time to reading the entire "official lowered outback thread", educate yourself, and decide the best setup to achieve the results you want.
Really it's not as complex as many make it sound. Any Legacy suspension bolts right up. If you install a setup and it's too low, get strut top spacers and longer bolts. Bilstein struts require JDM/specB front top hats. That's about all there is to it.