CAT D is the lowest damage for a car. Basically, the insurance company figure it's cheaper for them to give a cash settlement rather than pay out for repairs/a replacement (which would also include the cost of towing, courtesy car, inspection etc) but the car can be repaired fully and returned to the road. However, some insurance companies may charge a higher premium.
Best thing to do is test drive the car (make sure all the gears work, it can get up to speed, the clutch isn't jerky or slipping, breaks work well etc) and really inspect it if you go see it. Like under the bonnet, oil, coolant, the paint, bumpers, wing mirrors, even under the car! Consider getting it checked by a mechanic (but you'll need to pay for this)
Ask tons of questions about it: What damage did the car sustain? Where and what work was carried out? What parts were replaced? Check the service history.
Hope this helps.