I purchased the avital 3100LX model. It does not include the siren and that's fine with me since I prefer the horn. It is the equivalent of the Viper unit described on Lutz's website. The viper and avital are made by the same company - so essentially it is the same unit (again that is also mentioned on the Lutz website).
About $50 on amazon and includes 2 remotes. The remotes are silly expensive, if you wanted to buy 2, it's probably better to just buy a second complete alarm just to get the 2 remotes and have a spare alarm if ever needed
cheers,
george.