500D for beginner to enthusiast.
50D for enthusiast to amateur professional.
5D Mark II for amateur professional to professional.
1Ds Mark III for professionals.
It's the glass you need to worry about, though. That's where the money goes.
Any of those would make an ideal imaging platform, with the bottom two having the benefit of being full 35mm frame sensors, as oppose to 1.6x crop sensors.
For a rockin' astro imaging platform, I'd go a second-hand 40D or a 50D.
For comparisons, head on over to
dpreview.com
Regards,
Humayun