Well, you are going to get the mount either way, so get the mount
Re the scopes, I would defork the LX90 AND pick up a 2nd hand ED80.
A 2nd hand ED80 can be had for around $300 or so, A dovetail to suit the LX90 is $110, so it doesn't come to much. i.e. about $500 on top of the cost of the mount, and you can use both scopes.
Don't worry about an OAG initially until you have come to terms with the new mount etc.
When ready to look at an OAG, I would look into the QHY5L-II (or the chinese equivalent) as it is way, way more sensitive than the SSAG, it is up there with the Loadstar for sensitivity for guiding (I owned both). The SSAG isn't really up to the job for the LX90.
Dovetail:
http://www.bintel.com.au/Accessories...oductview.aspx
It isn't too hard to modify the LX90 to fix the mirror flop problem, if you are comfortable pulling it apart. (There is usually very little anyway on the 8")
A simple focuser mod might even be enough
http://jan.eaglecreekobservatory.org/focuser.html
You will probably get something back (at least a few $$) by selling the LX90 mount itself.