I would agree on 2 points...
1. Changing to an SSD only because the computer is slow on startup is overkill. However, once you go SSD, you won't want to go back to a HD.
2. Shutting down every night is overkill - if you do not also unplug the power cable every night, you are probably saving between $0.50 to $1 a year in electricity by shutting down vs allowing your computer to just go to sleep.
However, to provide any assistance on a slow startup issue, we need to know either:
A) How many seconds from pressing the power button until the login window appears.
B) If you have your computer set to boot direct to the Desktop without logging in, for testing purposes, you need to remove all items you have set to start at log in, then time how long it takes to reach a functioning Desktop from the time you press the power button. (You do know, you can add them back after you are done testing.)
While testing, you will do a full shutdown, not a restart.
With item "B" above, any additional time your personal startup requires would be due to the time to load the items you have set to start at login.
With an accurate report of time to the Login Window or time to Desktop, I can possibly provide further recommendations on decreasing the startup time.
And... there are no "needed" items at Startup - because they can all be started manually after you log in. Most of us do have items we "want" to load at Startup. I have 4 myself currently and that number increases and decreases from time to time.
I did not check, prior to posting, but techiesteve is correct as to the available drive sizes for a 2013 27" iMac - also please advise what drive you have.