The 100mm Planar is indeed more expensive than the 80mm, but even though the camera is in very nice condition the price is quite high, even without a mal-functioning shutter. (Something similar with a black T* 80mm should be about £350-450, at least in my mind. So you are paying more or less £500 on top of the 80mm to get the "famous" 100mm.) I'm not trying to rub anything in, but even though the 100mm is supposed to be extremely sharp, it's optimized for infinity. I.e. it's only sharper than the other lenses at long distances, like airplane photography etc. Having said that, the 100 still performs well at closer distances, but not better than the 80. Another reason which I find more appealing is to have your focal lengths more evenly spaced. I.e. having a 38-60-100-150 combination vs a 50-80-120-180 combo.
If you go ahead with the purchase, you must know that most of your money goes into a lens with a currently broken shutter. As TheFlyingCamera points out, some spare parts for C-type lenses are hard to source. Please do see to that the seller either have the lens CLA'd before going through with the deal or that you have a money-back guarantee if the shutter is broken.
(My personal choice would actually be a 110mm as I shoot with a 2003FCW. But that is because I'm in that "wide-open" phase of life at the moment. We'll see if I find one at the right price before I recover.
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//Björn