Hi tracy and welcome to the forum
Your credit rating will be impaired whilst you are subject to an IVA, but there are probably still a number of lenders who will provide finance facilities to you, so long as you have your Supervisor's express permission.
You will also need to ensure that the new car payments do not disturb your ability to fund your IVA, in which case your Supervisor may need to approach your creditors for a formal variation. This will only be done if it is essential that you replace the car, and not just because you fancy a newer model!
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk