3 things

Do you mean.. Can you look like a ghoul?... Yes. Can you turn into a ghoul?... I don't think so, I hope not. to look like a ghoul you need to help the ghouls move into ten penny tower, I did it by talking [ speech ] and dident kill anyone.

Sorry I don't know about the rifle, and I can only get to level 20 and then no more exp, points are gained.

I saw in the guide that the rifle was on a body in a diner in downtown and all it is is a cheap mask that doesn't even look like it but it fools the ghouls so...

1. No. If you get to 1000 rads, you die. There's no way to become a ghoul. You get a ghoul-like perk if you complete the optional objective in the Wasteland Survival Guide's Radiation Section.

2. The best Assault Rifle in the game, and a unique version of the Chinese Assault Rifle. It has a larger clip, (36 instead of 24) does more damage, and can be repaired with regular Chinese Assault Rifles. Here's how you get it:
Go to the museum of technology. Straight ahead after you enter there should be a terminal. It should have an option that says #000, which is a message from someone named Prime. It's a message he left for his partner, saying that he left the usual "breadcrumb trail". Now exit the terminal and enter it again. There should be a #001 now, but if not, search the other terminals around where the first one was for the first number. I'll get to what to do once you access them in a minute. The next should be right after you enter the West Wing. Or wherever that door after the Vault area takes you. To the left when you enter should be a couple terminals (Top Floor) one of those should have the next one. The last is in a terminal at the base f the rocket. If it's not at the base, check the others spiraling up around the rocket. You know you have a terminal when there's an extra option that says #00(Number) There are 3, not including 000. For those three, you're given the instructions, "Good Luck" and then three or four numbers. Sound impossible? It's not. The hint is actually in the guy's name. "Prime". So choose the prime number of the bunch, then number that can only be divided evenly by itself and 1. Once you get all 3, the terminal you finished on should have a final message, which will give you a password for a safe, and a map marker (Jury Street Station) where Prime told you to meet him. Now, go to the planetarium. If you entered from the entrance area, go through the door on the left, and then the door in the right in the hallway. You'll end up in a room with a safe and two terminals. Unlock the safe with the terminal closest to the safe and inside you'll find some drugs, 200 caps, and a key to a weapons locker on the opposite side of the planetarium.
Now, head to the Jury Street Station. When you get there, go into the diner and hop over the counter to find Prime.
Or, what's left of him. All of his limbs are torn off, so he won't mind if you help yourself to everything on his body, including some meds, 300 caps, and the Xualong Assault Rifle.

3. Level 20 is the level cap. Once you reach the level, you can't gain any more experience.

Hope This helps.