Old Website to WP Site

I have a client that has an dated existing site that wants to move to wordpress.

He has about 400 pages on this old site so a full migration is out of the question. Some pages are still getting a lot of hits on daily basis so he doesn’t want to just pull things down.

What would be the best way to go about this? Keep his existing site as a subdomain and have redirects to the old stuff, almost like an archive? I mean I should be able to have somthing like oldcrap.hisdomain.com any feedback woud be great!

I just have no idea how I would even go about this.

Thanks!