Insurance wise, if you have a clean record, and crap like that, depending on where you live, it may only be $150 a month or less.
The ratings on the original sticker for the SVX are 17mpg city/24mpg highway. Drive it easy, and your mileage will be a little better. I've heard of people getting high 20's by driving it nice and easy, not bad for a 6cyl. Now, don't let this discourage you, it's not going to get Civic type gas mileage, because it has a 3.3L engine, and it's a heavy car.
AWD is always active, it just sends power to the rear wheels when it senses the fronts spinning. About the "manual" button, it is for starting the car in 2nd gear when you need the traction, like on snow or slippery conditions. To use it, you push the manual button, and place the shifter in either "2" or "3".
The SVX does have 3 main flaws that you should know about before buying one:
1. Transmission. It is perhaps the weakest part of the car. It is known for having transmission problems, if you drive it hard, race it excessively, stuff like that. A transmission cooler is strongly reccomended if you do buy it.
2. Brake rotors. The SVX is a fairly heavy car (3500lbs.), so if you brake hard, brake at the last second, crap like that, you will warp the rotors.
3. Wheel bearings. The SVX is known for having the wheel bearings go.
Overall, SVX's are damn nice, just as long as you keep an eye on things, do regular maintenance, don't drive it hard all the time. Also, check out
http://www.subaru-svx.net/forum/ . It's a forum on a site devoted to the SVX and that forum will help you out a lot better than this one ever will.