To be valid an invoice has to contain certain things:
The Date
Your business name and address
A unique invoice number
Your VAT number (if you are VAT registered)
You can use the templates in Word/Excel for this or design your own.

You will need insurance and you will probably need to register as a trader with your local council.

You will have to get a HPI account - as a trader the liability rests with you if you take in and sell on a car with outstanding finance.

Membership of the SMMT adds some credibility but they want to see premises - not sure if you need a track record.

Selling cars will probably mean offering finance (there's often more money in the finance than in the car!) so you are going to need a Consumer Credit Licence - you can go to jail for not having this if you start selling HP etc!

And why choose Jaguar? Unless you have a long track record with these cars I don't see a great advantage to restricting yourself to the one make.