..... do you think the 50mm 1.8 would be a good one to add next?
A question that only you can answer really - a decision very much guided by how often you would need/use a 50mm focal length, and a prime at that. Bearing in mind that on a crop sensor camera like the 7D ( x1.6) this 50mm lens will give you an effect field of view ( FOV ) of 80mm. Additionally, you already own the 18 - 55mm ( 28.8 - 88mm effective FOV ) so you could also ask yourself how often, since it's purchase, have you used the 50mm end.
There is no doubt that the combination of the 7D and 50mm lens, both with very high resolving power (even more so the f/1.4 version) are capable of producing exceptionally high quality images in low light conditions. In fact, not all that long ago I was asked to take the photographs at a very important family gathering and purchased the 50mm f/1.4 especially for that reason. I didn't want to use flash and the combination of pushing the ISO value right up on the 70D along with the 50mm f/1.4 I was able to produce some remarkably noise free images. Since then, however, I've not used it once. For me cash isn't a problem fortunately and so I am able to have these expensive bits and pieces lying around waiting for a rainy day. From your post, this isn't so for you so whilst the 50mm lens is good I would first ask myself the question as to why you 'think' you might need it.
I agree with Forseti. I have the Sigma 50 1.4 which sits in its pouch in my desk draw and is only used for those special times when its needed. In a way, the 1.8 would have made better economic sense, but there you go. As for build quality, it doesn't mean a thing when you consider such a lens as the 1.8 can easily be seen as throw-away and another easily purchased to replace it. That's not so easy when you have paid out three or four hundred quid for virtually the same thing. Go the 1.8 route, you won't be disappointed.