Not stupid. At least if you ask me. I've been jailbreaking since late 2012. Every iPhone I've ever owned has been jailbroken, including my current 6s+.
So no condemnation from me on purchasing a jailbroken iPhone.
A couple of things to do first.
1. Get a computer capable of running the latest version of iTunes. You need to run the most current version, NOT the latest version that your computer can run.
2. Update iTunes to the current version.
For the restore, I am going to assume you want to wipe the phone and restore as new. We can try DFU Mode for that. DFU mode is a base level interaction between the device and iTunes. There's no GUI involved with the device itself.
So, here you go for that.
1. Open iTunes
2. Connect iPhone to computer.
3. Turn iPhone off.
4. Put iPhone in
DFU Mode The act of doing this will turn on the phone. Keep in mind, if you see a screen on the iPhone with a USB cable and an iTunes logo, you are NOT in DFU Mode. DFU Mode shows nothing on the screen (it's blank).
5. iTunes will detect an iPhone in recovery mode once you are properly in DFU Mode.
6. Have iTunes upgrade and restore the device as new.
At this point you can either set the phone up as new or you can restore from iCloud or iTunes from your own backups.