Good choice of PSU, if you check the PSU links in my sig below you will see how highly regarded Seasonic are
Tip for if you have never swapped out a PSU before, after disconnecting the power cord from the wall socket press the power on button for around 10 seconds to get rid of any residual charge left in the system, get yourself some tape that you can write on, stick a label on each of the connections from the present PSU identifying what they connect to, DVD drive, HDD etc, the 24-pin ATX power connector is hard to miss but one that does sometimes get forgotten is the 4-pin ATX power connector so be sure to stick a label on that, once all connections are in place, tidy up any unused connections using zip ties, helps with the air flow and removes the chance of anything jamming up the CPU cooling fan etc.
What about the chassis fan, one is worth investing in and it would be easier to fit with the PSU out as there would be less wires to get in the way while you work.