If you're thinking of buying the Tamiya Bismarck, you might want to look at the new Revell kit instead (this kit has been discussed previously on this forum - a search for "Revell Bismarck" will find more info - and there are numerous reviews online). Revell's kit has a much higher level of detail and fineness overall (and a single-piece deck!). I don't know how the price in the USA compares with that of the Tamiya kit, but in Europe the retail price of the Revell kit is only marginally higher.
On the other hand, if you already have Tamiya's kit, it's still very good considering its 30-odd years of age (as are the rest of their 1/350 series) and well worth building. A PE detail set would definitely be recommended, especially one which includes hatches etc. for the bulkheads.