- Does it void the warranty?
- Is it possible to go back to original OS?
- Would i still have all the original apps once i root?
Thanks in advance for your time.
Methinks you are confused about the definition of rooting
Rooting is simply adding superuser access to the device, allowing you to change things (such as install custom roms, which is what you are referring to in 2 and 3). Once you are done, the only change you will notice is the custom recovery (if you rooted using a method that installs one), and that's if you boot into recovery. So, your questions:
1. Yes (and I mean just rooting). It voids the clause in your warranty that voids if there are hardware or software changes to the phone other than the intended ones (not sure the exact wording obviously lol). However, I have heard that if you do have a problem and don't tell HTC you rooted, they'll still honor it. But I've only heard that, so take it with a grain of salt.
2. When you root, you will still be on the stock OS. If you install a custom ROM, you can revert back as long as you made a backup.
3. As long as you're on stock OS, yes. If you change, it depends on the ROM. I use Liberated Aria, and it imported all my apps/contacts/messages/settings. If you go a little more extreme (Cyanogen, for example), you will basically be starting over, like when you took it out of the box. All your paid apps will still be available, you will just have to redownload them.
I get the impression you are not to familiar with rooting (and if I'm wrong, I apologize), so if you do plan to go ahead, I would go to the XDA Aria forum and look for the NOOB Guide to rooting. This is the method I used, the directions are thorough, and the rest of the thread can help you with any issues you may run in to.
Sorry if that was more info than you needed, but I hope it helps!