You will love it- I sure love my '86 and '87 ST tourers. You can go to a 27 X 1 1/4 tire with the stock brakes, or convert to 700C wheels and use fatter tires and fenders- but you'll also have to fit longer reach calipers for that (older Shimano 600's come nutted and fit & work just fine, especially with KoolStop pads).
Or...go all the way with a 650B wheel conversion and Rivendell Silver or TekTro super-long reach brakes (they are the same, but the Riv's come with KoolStop pads), which will give you super-fatty tire/fender option, albeit with limited tire availability whilst on the road. And if you go 650B, might want to double check on that BB-to-ground clearance, especially if you're running 175+ crank arms.
As a side note, if anybody else is looking for a Tall Boy Touring Frame, I have a very nice white '87 Cannondale ST frame and fork. 25 incher, so a 63cm equivalent. It's a big 'un.