My S4 is in Otterbox case, but I think I have noticed it's been a little warm on occasion. As regards to using tethering it couldn't be easier, I just drag down from the top of the screen and click the tethering icon to turn it on, having previously set up the password. Devices just connect as for any other wireless network.
The phone can still be used whilst tethering is on, texting, phoning, browsing the net on your phone all works exactly the same, just your phone can also supply an Internet connection to other wireless devices via the mobile phone network. Of course you will need to check your phone supports tethering, if not there may be an app for it, also you will need a Sim which permits tethering.
There are some good PCs available for about £200 upwards, so it all depends on what you want, and what you use it for.
It is also possible to tether via a usb cable, but I haven't tried that.