You should consider some way prepared the semantic core. Page, in the first place, optimized for high-request and / or principal for SERP, for example, Buy car. Then, drawn relevant to this inquiry - the associative keywords - buy car online, buy cheapest car. The rest is up to you. To this the most reliable way to learn to read more and watch the issue. Ideally, the incoming page is optimized for 1-3 query.
Depends on the amount of content that is available on that page Try to keep the keyword percentage below 4% if you have 500 words and below 5% if you have 1000 words Generally you should not be kicked for keyword stuffing in these range