I had an '05 883 standard Sportster. As an 883 it was a fun bike. It had plenty of power around town, and wasn't working too hard at highway speeds either. '04 and later Sportsters have an engine isolating system that lets the engine vibrate all it wants at idle, but it really smooths out at highway speeds. I found the bike more comfortable than my Roadking, but that is just because I like the pegs under me like on a standard style bike. The custom has forward controls, so that isn't going to be the same for you.
When I first got my 883, it got around 55 or more MPG. After a year, I upgraded power with a 1200 kit, SE air cleaner, ignition, Dynojet, and SE exhaust. It was everything I liked about the 883, but with a lot more power.
As far as handling goes, it isn't a sport bike by any stretch of the imagination, but it could keep up with my sport bike riding friends when the roads were tight and twisty. When things opened up, they just left me in the dust though. As far as Big Twins go, there wasn't a stock one that could keep up with the 1200.
I love the way the standard Sportster looks. It traces its roots directly back to 1957. Plus, there are TONS of aftermarket parts for them. Bags and windshield are just the start. (Though I hate riding behind a shield.)
I am not going to say anything bad about the Kawasaki. My friend rides a Vucan Nomad and it is an outstanding machine. My first ride was a Kawasaki LTD440. It was a great first bike.
I just hought I would share my positive experience with my Sportster. Most people who trash talk them haven't ridden a modern one. They are so different than my old ironhead, it isn't even funny.
The 883 Custom is a nice looking bike, but to me the most outstanding deal is an '04 or later 1200 Roadster.