Long term SEO for e-commerce site with digital documents

We need SEO services for our website (http://www.manualscenter.com/). The site provides manuals in digital format for obsolete electronics devices (tv, audio, home appliance). We're looking for a long term cooperation and a person/agency that is willing to work for us for years. If our initial cooperation will be successful We'll rise SEO budget _and_ You might receive 1-3 more additional websites on similar terms.

What We need:
- on site SEO optimization,
- keyword research,
- white hat link building,
- suggestions what We can do better in terms of SEO,
- good links (in terms of ahrefs, moz score),
- as result of above improved traffic volume (with more or less the same conversion rate).

Please note that although I lack experience to perform SEO myself I'm able to verify quality of Your work (and I will).

What We don't need and won't pay for:
- black hat tactics,
- seo not in compliance with Google webmaster guidelines,
- link farms, spam, links from the same c-classes etc.,
- lack of results.

We receive mostly traffic coming from long tail keywords. Currently site has 7k monthly visits and there's no active link building. We expect to see significant rise in traffic after one month.

We're realistic and don't expect You to use only highest quality links with PR5+, but We also don't want You to use scrapebox and spam us with harmful links.

In Your proposal please write outline of what can You offer. Please also write what We can expect in terms of traffic (You don't have to be precise, but it would be great to read 'I expect the traffic to go up by %x within month/months').

Thanks,
Matt