A 900 mm reflector has a focal length of 36 inches or so. This means it has a speed of F/8.0 which is pretty standard. If it is a good quality mirror, you should be able to operate the telescope at about 500x before you begin to exhaust its resolving power. I would suggest two eyepieces to start. A 50x for wide field studies of bright open clusters and the Moon, and perhaps a 400x eyepiece for detailed views of lunar craters and binary stars. This means you will need eyepieces rated as 18 millimeters and 3 millimeters or so. Personally, for such a sized scope, I would stick with low power eyepieces in the 100 - 300 x range since they will give the best contrast and image clarity.