I'd look at the $300 investment from the standpoint of what your time is worth. I also spent 2+ hours removing the adhesive residue from the stock lip. Total bitch ache.
Replacing the spoiler is very straight forward, just a time consuming process. Kies Motorsports produces some quality how-to videos- here's one for this mod-
Here's a good video re. the diffuser... Looks relatively simple- just a few screws and tabs. Shouldn't give you much trouble.
Watch a few videos before you jump in. Take it slow. I think you're positioned for success if you elect to invest the time.
This stuff is addictive once you start. Post a few pics of the results. Good Luck!