Merging three Drupal installations

I have three Drupal 6 installations; one for a family site, this one, and one personal-interest site. When I initially set them up they were on separate hosting accounts. Now, because I've changed hosts and because none of the sites has massive traffic numbers, I've merged them onto one account. Since I've done that, my next step is to merge them into a single Drupal instance so that maintenance will be easier.

I haven't fully thought out all the ins and outs but the think the sequence needs to be:

1. Update all installations to current version of core and modules
2. Compile list of modules and themes required by all sites. To keep the installation as lite as possible, only modules/themes needed by all sites will go into the sites/all/... directories. The others will be put into site specific directories.
3. Move files into subsites files directories. That way each site will be using their own files directory rather than populating a single directory with files from all three.
4. Migrate sites one by one, double check setup, and repoint the root-directory settings for the site. Since all three are main level sites, I'm going to install all three as subsites and arbitrarily point the sites/default directory to one of them.

I've finished the first step. We'll see how the rest go.