Depends on how many users are going to connected to the network, if its for a small ircd, with approx 20 - 100 people, 50gigs of bandwidth is more then enough.
If you are looking at linking to a network, you will have to work out how large the network is, and can possibly grow to, as well as what type of user base there is.
Rizon, which has a lot of large channels, bandwidth requirements are a lot higher then say, a smaller network with smaller user based channels. Large channels scrolling constant data to around 2500 - 4000 people will eat up bandwidth quickly - but it all depends if those users are on your server.
Only advice is to work out how many people you think will be on your IRCd, and take it from there, you maybe better off getting an ircd shell from a provider which limits the amount of users, which usually works out cheaper.
Good Luck!
