by Reddawn36
Based on what you're looking for S3 sounds like the best mix.I'd say S3 > S4 > S2
Elder Ones I haven't played yet but Yog-Soggoth the consensus is on the easier side while Black Goat is on the harder side. Ihaqua may be too new and I haven't read much feedback.
If in US I'd recommend checking out AZ Gamer's Guild - they sometimes give some cart discounts and their shipping has been pretty timely.