Hi Steve. Go with the DMP for now. It will give you a chance to get your plans together. If you do eventually go for an IVA then, yes, as there are joint loans, it should be interlocking with your wife. However, if she can service the joints loans herself then she needn't join in. She will become soley responsible for the entirelty of the joint loans, even though they will also be included in your IVA.
Also be aware that Payplan do operate a policy whereby they take the HOUSEHOLD income to work out the IVA payments. So, with them, as well as paying the whole of the joint loan repayments your wife will also be expected to contribute to your IVA! Not all firms do this.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014