Whether becoming a nominee, will allow you to sign for a lease on your wifes behalf or whether you would need to be your wifes appointee, making you responsible for her disability benifits from the DWP and ultimately have the responsibility of the lease would be required by Motability.
It use to be the case, Motability would allow a dealer to attend your home (if they offered this service), so your wife could sign for the lease and again to enter the PIN. However I’m not sure if this is still allowed and highly recommend you speak to Motability and see what options are available on a case by case basis.
Alternatively you could lease or buy privately, even if its a Dacia and use part of your wifes PIP allowance to fund it, that you your are able to retain full control.