Hosted LDAP Services [closed]
Have you looked for Managed Service Providers in your area? That can be a good fit for getting a specific service and technical skillset that you don't have in-house.
I hate to point this out, but you say you're a business ISP and you don't have (and won't hire) anyone qualified to run LDAP. That sounds pretty bad to me, like if Amazon didn't want to invest in DBA staff - but maybe a MSP would fit the bill for you.
Symplified offers a cloud directory service that "provides . . . support for a wide range of LDAP and RDBMS systems as well as cloud services." eWeek provides an overview that may be useful.
Entic.net, a small company that appears to specialize in directory services, is beta-testing a distributed "Cloud DS" LDAP service. No details on the site, but they invite interested parties to contact them about participating in the private beta.
eApps offers an OpenLDAP VPS, publishes an SLA, offers a 24/7 support option, and locates its servers at a QTS datacenter. Unfortunately, this doesn't appear to be their core business. I do not know whether they provide management tools that meet your criteria.