I have an 120MM and a 174MM, both work very well in Ha. The 120 is probably a tad small in chip size but it would give a full disk view (just).
The 174 is a very nice camera, but those pixels are a bit large. I use it at 711mm which gives it a nice full disk view, but even then the resolution is not that good. The 183 would be a great camera for me - shame it doesn't seem to work for Ha.
Using
http://www.skyatnightmagazine.com/as...iew-calculator the 178MM looks ideal for your focal length, but I have no experience with it personally.