As already said, if you have the correct visa, then you should be able to book a one-way. If you don't yet have the visa, then technically you need an outbound ticket. But I think the chances are good that you won't ever be asked to produce them. However, even with the right visa, it's the check-in staff that would be most likely to raise the one-way thing and ultimately it's at their discretion.
Before coming here to work I read a lot of anecdotal reports of others in the same situation. Most were never asked about the outbound flight. I recently had two experiences of one-way entry (myself, then my family separately) where we were never asked about an outbound flight either. However, I did have a ticket in hand in the event that they did ask me.
If I had to do it again, I'd probably not bother with the one way and if the check-in staff had issues, I'd just purchase one on the spot. The ticket cancellation fees seem to be pretty standard (approx 1000 NT), so the actual ticket price doesn't matter much (assuming you have enough in your account to cover it).