If you use narrow band filters an achro is fine. You will need to refocus if you change filters.
If you intend to use RGB filters I am sure you will find that blue will give you the most grief as the deep violet will not focus at the same position as visible blue. Visible red and HA will also not be in focus at the same position but it is not as bad as the blue end.
I use a 150mm F5 Skywatcher achro for narrow band data. It has a Hutech FR/FF (7887) so has a focal length of 640mm and f/4.25.
Here is 16x2 min frames of HA at 1600 ISO before the cloud arrived. This is just a test. 2.5MB
http://d1355990.i49.quadrahosting.co...2070_32m_g.jpg
Bert