alright. The hard drive is easy enough. Take the back panel off with a couple screws. unscrew the HDD mount from the case, transfer the mount to the new HDD, and reinstall the mount with the new HDD in it.
Installing windows? they only sell Windows 10 now. It comes in a USB. plug in the USB and boot up the device with the new HDD in it. ''If it doesnt automatically boot up to installing windows, you will need to boot from the bios menu. '' to do this, power down the laptop, and reboot it. repeatedly press F8 until booting completes. this will let you choose what to boot from in the Boot options.
"Fixing" plastic isnt quite possible. if you want it looking nice again, you will need to replace the top panel of the housing. a housing set can be ordered online based on the model of your aspire. ebay, amazon, ali-Express, etc are good sites to check, but you may have to do some extensive google searching. Removing the panel is all clip and screw based. if it wount clip out easy, then there is another screw hiding somewhere on the housing.
The hinge is made of steel, and likely had the anchors broken off of the housing, so buying a new housing likely will fix that issue as well