I don't know anything about aftermarket units but I can tell you that the factory fitted alarm that came in mine standard is excellent and works seamlessly with the Tig. If it is available as a retro-fit that's what I'd be going for, saves any issues with warranties, compatibility etc etc. It doesn't have a proximity warning device though but it does have an internal motion detector and tilt sensors (these can be deactivated as required).

Cheers

Cheers