PPRuNe moved from the US because of relatively poor service from their previous provider in the form of a lack of responsiveness. Rather than just provide a hosting facility, DSC manages the server and its administration. The big performance gains enjoyed when the service was moved was not just because of a new faster server, but also a matter of configuration, the quality of the network connectivity and associated network components (switches, routers, firewalls, etc). As usual, generally you get what you pay for.
Mail is separate mainly for historic reasons. Indeed, only the free mail service is separate - we migrated the pprune.org mail accounts to DSC. There are free mail providers that will host your domain for free if you bring them new users - they hope to make their money from convincing your users to upgrade to higher specification paid accounts.
DSC normally only hosts for existing clients, for "good causes" or if requirements are too specialised for one of the mass-market hosting companies (and is therefore technically more interesting). Drop me a line with a bit more information about your requirements and I'll see if we're able to provide a competitive quote.