Hi I recently bought a new alarm with a mind to integrating with Homeseer. It's not cheap but I looked at ESP Infinite Prime and Visonic Powermax (I got the Powermax Pro).
As an alarm in it's own right it will dial out and SMS out (with the right bits e.g. GSM module). I bought it from these guys
http://www.tritonfiresystems.co.uk so you can see some of the options on their site.
You can add PIRs, door contacts, flood, gas, CO2, fire sensors. Have a look at
http://www.visonic.com for more.
With an XM10U module you can get Homeseer to read if it's alarmed/away/home/disarmed and create events based on it. In fact I've just posted up a guide to doing just that as it wasn't obvious (to me anyway).
If you want Homeseer to pick up the Visonic PIRs/transmitters then they work on 868mhz not 433mhz but RFXCOM (
http://www.rfxcom.com do a receiver that will read 433/868 or both.
I can't comment on some of the options above or compare them to Visonic but I've been really happy with the Powermax Pro.
Good luck if you want any more information drop me a message.