Is the ABS unit leaking? I assume it is if you're thinking of replacing it. I went with a remanufactured unit from A1 Cardone. I had to send in a core, so I bought one from a junk yard for $50 to keep from having to be without the Q for a couple of weeks.
It's basically a bolt-off, bolt-on deal. Three brake lines and two feeders from the master cylinder. Two electrical connections, and unbolt the holder from the car if you want to repaint it. It's been a while since I did it but it's fairly straight-forward. As far as flex lines - I just did those. The best advice I can give is to buy a good quality flare wrench to loosen those hard line to flex line connections! Check out this thread...
http://www.nissaninfiniticlub....30084
and get some speedbleeders to make quick work of the brake bleeding. Two SB7100 for the ABS and 4 SB1010 for the calipers.
http://www.speedbleeder.com
Good luck!
Heath