We have used UK2Net on quite a few occasions and had no problems with them - we do, however, go for the Dedicated Server option and perform our own administration of the servers. It really does depend on the project's budget - if you're after a full managed solution, then you can't go wrong with Rackspace - first class in every department !!

It does seem you are after a customized application, and not just a simple CMS. Try and have a look at bespoke applications specifically for estate agents - there are a few out there if you do a search.

If you are looking for a developer, make sure that the person understands the business domain and is not just simply a "coder". And make sure they are reliable !!