How does the AP900 compare to the AP1200 (if you have any experience)? Remember, the AP/Paramount mounts are far more expensive than the Celestron/Losmandy, which aren't cheap either. I'll probably end up imaging with a Deepsky Instruments 10" corrected RC within 2 or 3 years (probably sooner knowing me). I can't justify the cost of a RCOS setup, which I personally think are deliberately overinflated price wise. The RC market has had almost no competition for several years now, and I suspect that there's some serious illegal market collusion in play with price fixing the result. The best thing for the RC market is GSO's RC and Deepsky instruments imho (with the latter being far better optically I suspect).
I was planning on guiding with the Equinox 100 that I now have, but I might keep that for visual use and grab something like a WO Megrez 80 or 90. Again, I can't justify the expense of something like a tak or televue setup, especially for what would be a guide scope.
These are long term astro imaging goals for me, so depending on where I decide to go, will mean how much it'll cost me, and how long it'll take to get the funds together etc.