I will not try to talk you out of getting a large monitor. The reality is that the more people try to talk you out of it, the more determined you are likely to get to have one and "prove" everyone wrong.
However, a nile is the poorest choice of all the poor options you are considering. Niles are cute when they are babies. They run, hiss, whip, bite, and crap. It is cute. Then they get bigger. Most of them do the same thing, only with more force and more energy. Then they get to be 4-6 feet. At this point, if they manange to bite you, you will likely need stitches. If they scratch you in defense mode, you will likely need stitches. Explaining to mom, dad, and the doctors is unpleasant. A rare few niles calm down, most do not. Every potential nile owner "knows" someone who has a "tame" nile. That's wonderful. However, for every one of those "tame" niles, there are dozens - perhaps hundreds - that never calm down. Trust me, I've been offered more niles than I can count because it is too aggressive and too big.
Letting an animal free roam around a room or basement is not an easy adventure. At least not if you want the animal to live for a long time. I keep animals loose in a converted bedroom. It took me years before I was comfortable doing that and it is an ever changing process. A loose monitor will find the things you never thought to consider and will cause problems. Accept that as fact. That fact, in and of itself, is not the problem so much as your ability to correct the problem once it is found. I own my house. I can knock down a wall if I feel like it (and I have to some extent). You are 14 and most likely do not have that option.
You are 2 years away from getting your driver's license, give or take a little time. 2 years away from having the freedom to stay out all hours of the night with a car, significant other, and friends. Money will flow freely for all the things that 16-year-olds do. A large, money sucking, monitor will seriously inhibit your ability to do things with friends. Monitors are not like snakes. Monitors need to eat every week. A 4-6 foot nile will need a lot of mice or rats every week $$$
If you are intent on a large monitor species, get one your parents or whoever you live with now will be able to care for when you disappear for weekends, weeks, or do not have time. College, if it is in your future, is only 3-4 years away. Someone will need to care for the animal while you are away. My mom was a trooper. She cared for a caiman (while I was in college), water monitors, arguses, savannahs, and iguanas. She was truly scared of the arguses and very nervous about the prospect of caring for niles. None of that was fair to her.
Consider long and hard about why you want a nile or large monitor. Pick what is best for your situation now and in the immediate future.
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^x^ Bloodbat ^x^
Monitors, monitors everywhere
and all the food they ate.
Monitors, monitors everywhere,
their parents loved to mate.