Some notes on the older Cannondale ST's. I'm going to avoid the debate that has been raging on this thread about semantics and go with my experience and the experience of friends.

I picked up a 1990 Cannondale ST600 earlier this year in the largest size they made, 25" (63.5 cm). I paid $400, had been used lightly and was in excellent condition and came with all original components or stock replacements, such as tubes, cables, tires, etc. These bikes came with 27" wheels (the final year for that size before switching to 700c, at least on the ST600). I won't get into the details of the rest of the components and upgrades as they're not particularly relevant but I will say that I've swapped out the wheelset for a slightly newer 700c set. The rear spacing on these frames is 130mm which means that a standard road hub for 8 speed and up can be used, which is the way I went. The original hub is 7 speed. With the 700c rim, I'm able to very comfortably run 700x35 tires with fenders. I'm highly confident I can fit 700x38 with the fenders on and larger with no fenders. Note that running 700c rims on this frame makes for a funky alignment of the brake pads relative to the braking surface. I have the pad posts all the way down in the the cantilever brakes and still can't quite get them to contact the rim perfectly parallel as I can't get them any lower. However, at least with the Kool Stop Salmon pads I'm using, it doesn't seem to affect the braking quality, or at least I can stop fine.

A friend has an identical ST600 with two exceptions: His is a 1991 and the frame size is 23" (58.5 cm). 1991 was the first year of 700c rims on the ST600 so no brake pad alignment issues on his. He's set his bike up as a gravel/cross bike so no fenders and the largest tire he can fit. I know he was able to shoehorn a 700x40 in it. He want to get a 42 but I doubt that is going to happen.
He got his around 2 years ago and paid $350. Bike had been used heavily but is still solid with no damage aside from quite a bit of paint scratches. Most components had already been upgraded or replaced.

Both bikes have the original steel forks with all the attachment points for fenders and racks. Both of us enjoy the ride quality and handling.

Here is my bike setup exactly how I'm using now as an around town ride and commuter with 700x35 tires.



Here is my friend's bike how he is using it as a gravel and all around bike. The front tire is different and small because he shredded the rear so he swapped the front to the rear and got a used spare my brother had hanging around the shop to put up front.