Manual transfer of old company website from Joomla setup to WordPress

I’ve started work recently with a company who had their website set up in Joomla. By all accounts, it’s a cobbled together, odd-looking and unwieldy thing, and the owner has been advised by outside developers and myself to switch to WP. I’ve found a theme, set up a test domain and I’m ready to get to work on building a parallel site to move to later.

However, I don’t want to do a direct transfer using FG Joomla, as the categories, posts, pages, tags, media etc are all messed up on the current site. I proposed that we just copy all the content – the posts, the pages, the images, the details of products etc – manually and put it up piece by piece on the test site with the new theme. Extra work, but it seems to be the best option to avoid another mess and get a clean start with the new setup.

What I’m concerned about is the differences that will be there with regards to the database and URLs. Ideally I’d like to keep the main URLs (e.g. homepage, product page) the same, but the rest would be ok to change. (As it is, a lot of the URLs are ugly and it looks like the database is a mess, so I’m just in two minds about it)

How much trouble/work am I letting myself in for? How easy/difficult will it be to change from test.domain.com to domain.com when the site is ready? What pitfalls have I overlooked? Also, will it mess up the SEO ranking/PageRank score?