need to move all customers by then.
How many do you have to move?
Doing a Plesk >> Plesk migrate is quite simple so there should not be a problem for the host as far as that goes.
If you truly need fully managed and 24x7 support from a solid company, Rackspace.com is where it's at.
However..because you say that you are moving from a reseller account, I'd say it would be best to move to another. I haven't used them but HostNexus really does seem like a good host to me. Might be worth checking them out.