The interior of many MGBs have been neglected and can be in poor condition, some have been subjected to some creative modifications. The best situation is to find a complete, but well-worn, original interior. On most un-restored MGBs you should expect to spend a considerable amount of time and or money bringing the interior to original (or better) condition. Most items area available from leather seats, carpets to door cards however these will be expensive. A quality wood inlay dash kit can upgrade the appearance of dashes, although it will deviate from the original design. You can even upgrade your steering wheel, but remember the factory-installed wheels have a large diameter for a reason—MGBs do not have power steering!
An un-restored car is also likely to need a new soft top or fabric sunroof if it is a GT. The vinyl tops tend to get torn from improper folding and storage. The top frames are ruggedly designed and even those that do not raise and lower smoothly can be cured with some good lubricant. New soft tops are available now and in a larger range of materials and colours than when the car was new.
The interior of the car is where you will get the most out of your driving experience so it is worth spending extra, also if the interior is a bit scruffy then it is a good lever to get the price down.