The OP is asking for too much, for too little, and without even paying upfront. 15 500 word articles at $5 each, by itself, comes to $75, and that's under the rate that good writers charge. That doesn't cover the pictures, keyword research, site creation, let alone the traffic generation (SEO and/or viral).
This would really only make sense to do as a modified JV, where the worker got to monetize their activity as part of the agreement and keep the earnings (e.g., CPC ads, using paid links, paid reviews, CPA/PPD, CPM ads, etc) separate from the buyers' monetization.