When you talk about 'big improvements'....it really should be written "big" improvements. It's a lot like the newest cola drink with the "bold" "new" taste.
Frankly, Guzzi does improve the models, but those improvements are incremental. Oil jets under the pistons for cooling. I'm not sure it really needed that-I haven't holed a piston yet. The front balance tube gives a bit more low end, but I seriously doubt that you would feel any power difference between my '01 Sport and an '04 Sport. I went for a drive last night and was again surprised at how well my bike runs. I shouldn't be by now, but I still am. Slow learner, I guess. I think my bike must have been built on a tuesday, it runs so damn well.
The biggest issue with this particular bike is the casework- if it starts peeling (and it will), then it will look like crap. If you get a really good price on it, it may still be worth it to you. Since this bike is advertised as an '04 and it isn't, you should be able to back out of the deal. But just because it is an '02, doesn't mean it's cursed.
edit: if you hold out for another bike...keep a lookout for a model with ohlins suspension. It's apparently nicer than our proletariat suspension, and since it's used, you get the depreciation.