You need a Thatcham approved alarm- category 1 will give you both immobilisation and an alarm. For a convertible, you want microwave motion detectors, not the regular ultrasonic. These can be fine tuned such that you can leave the hood down but if anyone puts their hand inside, it will trigger the alarm.
For insurers to recognise it, they often want it fitted by an approved installer, who will also be able to give you a certificate to say it's a pukka installation. Expect to pay around £300-£350 for a system that will also give you remote lock/unlock. You may need to add a solenoid to the driver’s door for this- use one either from another 900 or a 9000. You will want the one with microswitch in.
It might be worth checking just how much of a discount you will get for having a Cat 1 alarm. If you're handy with car wiring you could do it yourself for under £200. I've used and can recommend Cobra systems and there's also Meta.
It is possible to buy a Cat 1 alarm and fit it yourself, but you don't get the certificate. Some insurers are fussy about this, others not.