First of all, you probably don't want to use too high of an ISO, because that will destroy your dynamic range (variation in the colours of stars) and brightens up random noise.
http://dslr-astrophotography.com/iso...canon-cameras/ is an excellent resource to find out the optimal ISO for a Canon camera (I know you have a Nikon, but there will probably be an equivalent guide if you do a bit of digging) which preserves the dynamic range and reduces read noise. Many cameras have a point in which they are ISO invariant, meaning that increasing ISO has no benefit except reducing dynamic range (which is also the purpose of that guide).
Like David mentioned, the focal plane of a camera is different to an eyepiece. Reflectors are designed in a way in that you can use them without any extension tubes or diagonals (unlike refractors or catadioptrics which require a star diagonal and/or extension tube to reach focus). This means that the focal plane is inside the focuser, and prime focus will be difficult. The only way you can combat this is to replace the focuser with a low profile one, move the primary mirror forward (and replace the secondary with it, to fully illuminate the field), or to use a barlow lens. But a barlow lens won't be ideal for deep space astrophotography because it significantly slows down your imaging speed (adding a 2x barlow means you have to take exposures 4x longer to capture the same amount of light). Another way is to get a reflector which is designed specifically for imaging, like the Skywatcher 150PDS. Since your mount is alt-az, you will also have field rotation issues limiting your exposures to only a few dozen seconds (this is especially problematic at higher focal lengths, like 1200mm).
My suggestion would be to start with untracked astrophotography using a DSLR and tripod alone, and move on from there. You can easily shoot the Milky Way and greater constellations. You'll be able to get a hang of acquisition and processing by doing this. From there, you can buy an additional astrophotography rig, whether it's on a small star tracker using just camera lenses, or a full on equatorial mount and telescope. Most people who do imaging and visual will have two separate rigs, because you can't really combine the two unless you have deep pockets
. The only kind of setup that I can think of which would work well for both would be a Schmidt-Cassegrain with a Hyperstar, but this particular combination will be not only expensive, but be difficult to use as well.
