Sigma as mentioned are fine, they OWN toad after all, but the toad ai606 is a ace alarm, the previous ai600 was better, me personally I would go for a C5T2, which is a 2-1 upgrade, still car 1 if fitted by correct people.
If you don't get any discount for the alarm fit it yourself, dead easy to fit easy to set up, take a wee while to do properly, but so long as ok around wires nothing hard.
Buy a NEW one though, the new ones have a much better digital link to the immobiliser.
I can email you all the settings changes, as you won't get these with the alarm.
You may want to enable the chirp feature for example, as they come in silent mode(arm/disarm) the chirp is actually ILLEGAL, on new installs due to noise pullution, but easy to turn on, with the 4 number code you will be given with the new alarm.
tabetha