A servicable flash unit can be had for less than $75 -
Yongnuo YN-560 Speedlight Flash for Canon and Nikon
For making professional quality portraits you would need some stands and light modifiers too -
Impact Digital Flash Umbrella Mount Kit
Nikon entry-level DSLR's that have the screw drive auto focus system in the camera - D50, D70, D70x, D80, D90, D7000.
Nikon entry-level DSLR's (compact) that
do not have the screw drive auto focus system in the camera - D40, D40x, D60, D3000, D3100, D3200, D5000, D5100.
Recommended reading -
Light Science and Magic, Fourth Edition: An Introduction to Photographic Lighting