Is there a tank size range that we have to stay within? If I could I'd do a 180 with a hopefully already big Manny. I have a 7in Manny in a 75 right now but he grows ever so slow. Like almost all Serrasalmus species.
I think a year would be ok. Just make sure to over filter and watch out for any aggression. A 55 wouldnt do for life but you could do a year probably. A better fit would be a 75 since you get a better footprint. I would suggest an irritan if you can find one for a 55 for long term.
if this is gonna be your first venture into piranhas, i would probably stay away from the more rare expensive stuff, there's nothing better at turning people off from this hobby than losing a $150 fish during their learning stage.
Get a gold spilo/mac. They are fairly cheep and are almost always aggressive. I have a 4.5 inch one that I have had for only about a week and it is a crazy finger chaser and also hand feeds. A 55 gallon tank would be plenty for life.