Need some advice about purchasing a used Cannondale
Are you physically near the seller?  The single best thing you can do is go see it, inspect it closely, and then get a leg over the bike.

Test Ride  That way you'll know instantly if its a "no-way" fit bit sitting over the bike.  A good  seller will let you take it for a ride too.  Ideally an hour, with some mixed surfaces, and a small climb/descent to get an idea how it goes.   Its common to leave something behind, like your drivers licence or similar ID as collateral, or sometimes the seller might come for the ride with you.

Components by buying used you're going to get an older groupset. And it will likely have some wear in it.   9 speed parts are available, but you might not find high end duraace or ultegra level.   105 is a perfectly good workhorse generation and even tiagra and sora and claris are useable.   I might think twice about spending money on non-branded stuff, or tourney groupset.

Brakes Most modern bikes will have dual pivot rim brake calipers.  These are superior to single pivot calipers.   But expect to replace brake pads basically straight away.  

Wheels Start by looking at the brake track.  Ideally it should be straight and not scalloped/curved.  Some scalloping will be evident on most wheels, but too much and the wheel is worn out.   The rims should run true, and all spokes should be present.  Bonus if you get some spares with the bike - they can be pricy on these low spoke count wheels.

Transmission triple chainsets work okay, and are a necessity for some of us.  There are snooty road riders who look down on triples as being for "bad climbers".  In the same vein there are 1x users who do the same to double-chainring riders.   In terms of hills a triple generally gives you more grannie gears at the low end for going up big long grades.  If your commute has hundreds of metres of climbing, you'll appreciate it.  For flat use, you're likely to stay in the big chainring all the time and only change gears at the rear.

It should change gears smoothly and reliably.  A little bit of clatter means it needs a tune.  A lot of pressure required to change means it needs new cables.  Can be a bargaining point.

Tyre clearance Modern trend is toward wider tyres at lower pressure.  That this bike has 23mm tyres means only that it will fit 23 and smaller.  You might find that a 25mm won't fit safely.

Tyres are kind of irrelevant.  As long as it has some, and they're not dry rotted you'll be okay.  These are consumable items, so when you start riding more you'll figure out if they're suitable, or if the next ones should be larger, or a different tread.  

Commuter features The yellow colour is good - there are too many black bikes that blend into the background, which is a bad thing.
Depending on your climate and commitment level, items like mudguards/fenders, rack, lights, and reflectors will be required. I doubt that bike has mounts for full mudguards, nor space under the brake caliper for the guards.  But then if you live in a dry place or if you have a wet-weather option, then this may not bother you.
A Rack is for carrying stuff to/from work.  You might prefer a backpack, its up to you. 
Night riding will ABSOLUTELY require lights and reflectors, of which this bike has none.
Security too - even though its 18 years old, its still stealable.  You'll need to park and secure it safely, or bring it inside at work.



I personally own a `90s aluminium cannondale road frame, and the tyre widths is probably the worst point, followed by road buzz.   I can only fit 25mm on the front and have to do naughty things to get a 28mm on the back wheel.  Even then it risks rubbing and wearing the inside of the chainstay.

The road buzz comes from our road surface, which is chipseal (small sharp pebbles embedded in tar) and it can make one's hands go numb.  My handlebar gopro goes poorly, and worse at night because of the vertical shake.  On other bikes it works much better on the same roads.



Is it worth $xxx ?  Well you need to look for other bikes in your area and see what they're priced at, and compare that to the cost of a comparable new bike.  

The new cost of that bike is irrelevant - its a nice bike, but its not a super highend race bike, while also not being a BSO from the supermarket.

Personally I think horizontal-top tube bikes look "normal" and the modern slopey-frames look like a dog scootching over the carpet.  But that's my opinion.  If it were taller I'd ride it.   And remember any bike beats walking.