I have a Smartrac s5 tracker with no tag no start functionality, fitted 8 months ago. Had no issues with it, the only drawback of it is that it takes 20 seconds or so to sync with the tag before you can start the car. The tag is a credit card that can be easily carried, other systems tend to have the keyring type tag. I used infiniti tracking, and spoke to a guy there called David who talked me though the options before I went with the Smartrac. https://www.infinititracking.co.uk/