Hi,
An all zone iLink fare is £16.50 plus £1 if you need to buy the smartcard.
You can't normally buy a card in Bangor station (check my username for how I know

). You can top them up anytime the station is open and there's someone working there. The ticket office opens early and closes late.
If you need to buy a card, you can book it online in advance and have it sent to you. You don't have to pay the £1 for the card if you do this.
If you want / need to buy the card in NI there are surprisingly few places to do so. I think you can get them in the main Great Victoria Street train/bus/coach station or at the Translink desk at the back of the Tourist Information office on Donegal Square. Pre Covid you could also buy them at information desks at the airports but I've no idea if that's possible right now.
If you're travelling to Bangor train station you can use the Bangor town buses to get there - it's included in the iLink daily rate.
Another tip is to sit on the right hand side of both trains - the view's are far better for the journey you mention.
All the best,
Mr Toad