alarm retrofit

Hello friends, my friend recently bought Vw Passat b5.5 without alarm(belive it or not), and he ask me can I recomend good one.

I dont know very much about this topic, but I know that this is right place to find out anything about cars, so Im asking can someone recomend good solid alarm?

All in all, we saw plenty alternatives, from cheap one to really expensive but is there really difrence, what to look for when buy and retrofit car alarm?

Tnx as always for really helping every time I came here.