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I think the notion of WP based sites doing well in search comes from the fact that it makes posting content very easy. So you could end up with more content, and better content (ie with images and image galleries/videos) compared to hand coding everything Also because the market share of WP sites is so high (around 20% of world's sites run WP) google is probably optimised to crawl and evaluate that structure well.

But WP suffers with a few faults out of the box depending on template, for example category pages/tag pages spit out the whole posts which causes duplicate content. Some SEO plugins alleviate that problem. Also another problem i've heard is of stale content, where an old post becomes more and more buried and many levels deep further from the homepage. This only applies if you are planning to use WP in its blog format rather then use the Pages functionality.

I personally don't believe numbers in the domain make any difference, but dashes might. This is due to more spammy domains having long dashed list of keywords in them.

Take a look at this piece on Moz, at the bottom you can see the consensus about  "domain has numbers". Virtually non existent effect observed: http://moz.com/search-ranking-factors

That is a very impressive list of clients, however from a UX point of view it's far too many to list on one page.

Instead why not have a dozen or so featured clients which have more in depth case-study each explaining how you approached each project. Then the rest of the clients can be a non-linked list. Maybe you can show a small description on hover of each client.

That would satisfy the SEO aspect as well. You would have a few strong case-study pages, and the rest of the content will be indexed also, just all on one page. Hope this helps.