Don't bother with an alarm they are a waste of money. As long as it has an immobiliser, which they all have to have to pass the SVA, you will be fine.
How often do you hear a car alarm and go running to see what it is? All they do is drain the battery and cause electrical problems. Anyone who can get past an immobiliser can get past an alarm, so they're not even a deterent. Doesn't help the insurance for a lot of insurers either.
(Does it show that I hate alarms?
Rant over!)
PS Re the sat nav - I would take it out and stick it on Ebay, the sat nav and TV will both be useless and can't be converted. I had a nice Pioneer one in mine and it couldn't be changed.