Personally I wouldn't buy an alarm system operating on the 433 Mhz band. This is a very crowded space, all the cheap/fun consumer electronics stuff operates on this band, your stuff but also all the stuff from your neighbors. Primary reason, there is no license needed for manufacturers and hence easier & cheaper for them to use. Vs other frequencies such as 868Mhz.
The 868 Mhz is license-restricted and used by all professionally installed systems.
Regarding Visonic, the system should be a Powermax system. Either Pro or Complete. But perhaps there are now other Visonics which are able to be integrated with Homeseer/RFXCom, I don't know. Search this forum for much, much more detailed information on the integration possibilities!
For reference, see enclosed my UK cost for my system (feb 2011). I had not looked at this list for quite a while. It confronted me with an outstanding ToDo; to finally activate my Powerlink with Homeseer...
Good luck!
Olof