SSDs are getting faster than SATA can deal with anyway (and googling suggests that your laptop has SATA II, your drive's probably quicker than it can handle). So yeah, getting a slower (relatively) one is smart since you get the same effective performance for less money (unless you're using M.2. or PCIE or something).
Easiest way to scale your CPU back in Mint is to use this.
If you want to get your hands dirtier, then have a read of this.
To dick around with power saving stuff in general in a finegrained way you want a package called laptop mode tools. There's *lots* of stuff to configure (if you want to, like). Good documentation here (it's for Arch but it'll work the same in Mint).
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