Something alot smaller, and less dangerous. I say this because with monitors you dont measure cages in gallons, you measure monitor cages in feet at the very minimum, yards, or meters, monitor especially big ones require very very big cages and in the case of niles they should have swimming pools to swim in, they are the second most aquatic monitor species. The species si very inexpensive to buy, they are very pretty but very very expensive to PROPERLY care for. How many do you know have a 20ft by 10ft by 6ft cage? I mention these sizes because the pool to swim for a 5-7 plus foot monitor, and area to move as well properly bask and adequete substrate, etc.
As far as attitude, the typical nile is not a handleable monitor, they are very very nervous, high stress, defensive, bitey, have big sharp claws, and dont hesitate to slap you with almost 2/3rds of their body (their tail). Not to mention their common habit of "power-crapping" all over you. I made that leap to get one and the monitor suffered as well myself years ago when it didnt eat much, hid alot, was completely unhandleable, and I didnt understand why. Mainly the reasons for it was my bad choices, as well it being typical of nile monitors.
If you have the space, the drive to build it what it needs, and the dedication to keep a dangerous, unhandleable monitor its whole life, then by all means go ahead.
Good luck, there are a few rare handleable niles with decent or good attitudes, just dont interpet playing dead for handleable or sick and dieing for handleable or tame.