I know that the idea to use Wheel Motors is very attractive for electric vehicles due its simplicity. And actually they work very well on bicycles because they are very efficient vehicles.
Normal street cars are everything except efficient (Too heavy and bad aerodynamics) so I do not think that Wheel motors are the best solution for a conversion. May be as commented here they will be enough for a hybrid conversion.
Anyway my opinion is that Wheel Motors will be on future electric car as soon as we have produce lighter and much more efficient cars, that will happen quite soon, energy and raw materials crisis is hitting all society levels already...
Technically talking I suggest for a DIY Wheel Motors to use industrial torque motors (quite big diameters and high poles count)
In order to give you some help I have been looking on some motors manufacturers and I think I have found some that will meet the needs of an electric vehicle.
This company provides them without housing so, if you have access to a good metal workshop will be not difficult to make the appropriate housing.
They are in-runners witch is very good because will simplify the cooling, actually I think if the motor is well matched to the duty (oversized) then only air cooling will be enough.
Looking into the list I think the model MF0410015B with around 30Kw peak power and 325Nm peak torque will do the job.
Although as I commented before I think that this motors will work really well on a light and aerodynamic car but not sure if they will perfrom really well on a normal street car.
Hope it helps.