I suggest getting a decent body with good noise reduction. You will want a wide lens, the 24-70 2.8 will not be in your budget. I would think the 18-105 will be. The 24-70 would be nice, but since flash is a must I would shy away from shooting wide open anyway. You will want some depth in your shots with people dancing. The 50mm 1.8 will be of little use.
So at 50mm, shooting at 8' away at an aperture of 1.8 your focus will basically only capture 4" in from and 4" in back of your focus point:
[TABLE="class: resultsTable"]
[TR]
[TD="class: textClassResults"]Subject distance
[/TD]
[TD="class: textClassResults"]8 ft
[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="class: textClassResults, colspan: 2, align: left"]Depth of field
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Near limit
[/TD]
[TD="class: textClassResults, align: left"]7.74 ft
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Far limit
[/TD]
[TD="class: textClassResults, align: left"]8.28 ft
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Total
[/TD]
[TD="class: textClassResults, align: left"]0.55 ft
[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="class: textClassResults"]In front of subject
[/TD]
[TD="class: textClassResults, align: left"]0.26 ft (48%)
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Behind subject
[/TD]
[TD="class: textClassResults, align: left"]0.28 ft (52%)
[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Hyperfocal distance
[/TD]
[TD="class: textClassResults, align: left"]230.3 ft
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Circle of confusion
[/TD]
[TD="class: textClassResults, align: left"]0.02 mm
[/TD]
[/TR]
[/TABLE]
Where as shooting at 18mm at 8' on f5.6 will give you almost 4' in front and 43.7' behind your focus point:
[TABLE="class: resultsTable"]
[TR]
[TD="class: textClassResults"]Subject distance
[/TD]
[TD="class: textClassResults"]8 ft
[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="class: textClassResults, colspan: 2, align: left"]Depth of field
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Near limit
[/TD]
[TD="class: textClassResults, align: left"]4.34 ft
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Far limit
[/TD]
[TD="class: textClassResults, align: left"]51.7 ft
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Total
[/TD]
[TD="class: textClassResults, align: left"]47.3 ft
[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="class: textClassResults"]In front of subject
[/TD]
[TD="class: textClassResults, align: left"]3.7 ft (8%)
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Behind subject
[/TD]
[TD="class: textClassResults, align: left"]43.7 ft (92%)
[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Hyperfocal distance
[/TD]
[TD="class: textClassResults, align: left"]9.45 ft
[/TD]
[/TR]
[TR]
[TD="class: textClassResults"]Circle of confusion
[/TD]
[TD="class: textClassResults, align: left"]0.02 mm
[/TD]
[/TR]
[/TABLE]
I am not telling you to substitute good focus with a larger aperture to compensate, merely that you will be happier with your shots to have, say one person dancing, and their partner behind them in focus as well.