The SVX was produced from 92-97. Sales decreased each year, so there are more 92 models than any other year. Very few 97 models.
This should provide you with lots of useful info:
http://www.cars101.com/subaru_svx.html
Exterior styling is basically the same for all years. Starting in 94, they added SVX badges underneath the rear window on each side. You can add those to 92/93 models if you want. They are about $15-20 each. 97 models have a different front grill. Beside color changes over the years, that's pretty much it from an exterior standpoint.
ODBII is an emission control thing (so was ODBI). An ODBII SVX will have a socket where the coin holder is on a ODBI model. A ODBII tool can then be plugged into the socket to easily reteive error codes. Very cool if you have the diagnostic tool. Not so ccol if you don't. ODBI cars rely on flashing lights to tell you the problem (you count the blinks and look up the code). Don't need any special tools to do that.
Somebody else whith more knowledge can speeak to the transmission upgrades performed thoughout the years.