As has been said, Cat D does not need a VOSA check. That is for Cat C only. Cat B can only be used for parts - and the chassis put beyond use, Cat A have to be completely destroyed - no parts to be used.
As for buying, a Cat C or D should be 25-30% below the equivalent HPI clear car. If not, then it's too expensive, as it is more difficult to sell on, and will have to be discounted by a similar figure then.
If the sums work for you, then you need to check out why the vehicle was written off. This is where a Cat C can be better than a 'D' as it has had a check before being allowed back on the road. Pictures of a car with the accident damage can help, as well as documented repairs and progress pictures. Stolen Recovered, or light all over panel damage is generally pretty good. Steer clear of anything that has heavy damage on just one corner, or anything that has been flood damaged (however tempting the perfect bodywork looks).