What are some of the more common narrowband imaging filters that you'd recommend for purchasing (e.g. Ha/Sii/Oiii)? I haven't got any at the moment so I'd like to start off by purchasing ones that will see frequent use for DSOs.
You've picked the most common three on the market. The only other one that you're likely to come across without going custom is the NII filter. A 5nm Ha filter contains the NII emission so you need both to be 3nm to isolate them from each other.
You can get filters like a Hb but they're usually not "narrowband" as such as they have a fairly wide emission and include OIII.
You will get a lot of use out of Ha by adding to galaxies and nebula. For galaxies it brings out the lovely red knots in the arms and for nebula it gives a fantastic luminance layer.
UVIR filter can help with getting good luminance layer for use in combination with the common narrowband filters mentioned, when you merge channels.
I use HA,OIII,SII & UVIR.