you are very close to using your phone again already and you don't even know it.
Just try the recovery, go to the fastboot option and click power while it is highlighted. From there, you will see a list with hboot, recovery, etc...
just click power while recovery is highlighted.
Since your phone says unlocked already, it is likely the previous owner put twrp recovery on it.
From twrp recovery you can install any rom zip file just going in the install tab, or any nandroid just by going in the restore tab and choosing.
There are a lot of good pages with roms in the forums at xda. I would suggest doing a full backup using the backup tab in twrp first before you install anything.
Those top 4 tabs in the TWRP recovery will have your options. Install, Wipe, Backup, Restore.
You can download tons of roms, and other people's nandroids. When you make a backup, just click backup and swipe, it will do the rest and insert it in a folder in your storage, which should be accessible while your phone is in TWRP recovery, if you hook up via cable to your computer, and look in the internal storage -> TWRP -> backups -> (phone specific serial #) --> NANDROID LOCATION (place any nandroid here, or you can copy your nandroid backup from here)
Once you have these things straight in your head, you should be able to operate your phone via TWRP.
The Nandroid backup is a folder containing several files (9 or more, system data are the two biggest ones, also includes boot, (always 16.3 MB) and others )
The downloadable custom roms vary and are always .zip files around 200-400 MB usually. They can be installed via the TWRP recovery after you wipe the top four options in an advanced wipe (again I mention make a backup first) This means check the boxes for Cache, dalvik Cache, Data, and System and then swipe and it will wipe them.
So once you did that, you would restore a nandroid backup or install a custom rom zip.
Then you can power up your phone by going to the bottom right tab in TWRP (REBOOT) and clicking SYSTEM.
If you don't have TWRP it is not hard to install via Fastboot. The TWRP can be downloaded. The current version is 3.0.2.0. it is phone specific so make sure you get the TWRP labelled for your phone, It is flashed via fastboot (a small file which you download along with adb and put in a folder on your computer and execute once . it will flash a command prompt for less than 1 second and you are good to go from that folder where it is located) Usually the fastboot and adb have a pair of .dll files with them...downloading them should be easy to find and only a MB or so.
You can find the fastboot fr your computer and phone from the HTC website, which you should make an account and begin the process of unlocking the bootloader (even though yours is already unlocked) and it will take you to a fastboot for your computer and phone to download. This along with the adb will allow you to EASILY flash the TWRP on your phone via the >"fastboot flash recovery (recovery name.img)" command (where recovey name is the name of your TWRP image, stored in the folder with the adb and fastboot exes and the .dll files). You open a command window in that folder by holding shift and right clicking somewhere inside the open folder (in windows explorer). Always operating from that location on your computer to your phone which remains plugged, and on the fastboot screen you said you have available on your phone.
Hopefully you go to recovery and it's already got TWRP on it, but if not, you can easily flash this TWRP found on the internet (about 10-20MB)
Of course, you didn't list that your phone says fastboot and etc (which should show below that Mar 18 2015 date...)
If not then I can't help you...
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