It's really easy to set up an inline diffuser on the fluval 07 range, I have done it on a 307 after having a in tank diffuser and you won't look back. You just need to buy 16/22mm tubing and cut it once extra (you can trim the edges of the ends to make it a little easier to attach). The one thing i regret (and am going to resolve sometime soon) is not putting the diffuser closer to the actual filter outflow. But I would definitely recommend going for inline over diffuser with only an extra 10 minute job and like £5-10 for pipe.
As for where to get co2, i found a local gas place (Andy's Gas in Bristol) because ultimately I wanted ease of refill, and I just got a dual stage regulator off of amazon which works perfectly, however it's really not much cheaper than going co2 art so either or really