If in fact your big old scope is in fact tacking well in Alt-Az, you can grab a picture. Your limitation will be what is called field rotation. So, you need to choose targets and settings that combat Field Rotation.
Before you start, check this out:
Now, I am going to assume you are not guiding. (The article quoted above assumes guiding.) You cannot just follow the suggestions there.
But, whatever you choose as a target needs to be something that will suffer the least field rotation. The closer you are to the celestial equator and to the meridian, the more your scope will experience field rotation. So, choose a target that is as far north (or south)----highest declination numbers) as you can and as close to the eastern or western horizon as possible. Of course, try to keep it above 20 degrees altitude to avoid other problems----imaging at a horizon brings bad seeing, light pollution, and all that.
Field rotation shows up more in stars than in nebulous objects. This is just because nebulosity is fuzzy, and stars are pinpoint. Pinpoints show streaks sooner. So, choose things that have more nebulosity than stars. (of course, no matter what, you will have stars). On the other hand, stars are a lot easier to get pictures of. They tend to be a lot brighter per arc second. So, choose things that have more stars than nebulosity. (Yeah, I know I went both ways on that........but those are your choices.)
And, yes, you want bright objects. Those that show lots of light per square arc second. You cannot tell by just using "magnitude" of a target, since that is a composite of all the light in the area that comprises the target. YOu want high "surface brightness" targets.
At 1225 focal length, you have a medium long scope. So, choose things that fit your field of view. To check out your Field of View, go to Telescopius.com, and use their telescope simulator with your scope and camera. Then, page through the possible targets.
So much for targets.
As far as exposure, you have to remember that the big problem for you is field rotation. This shows the longer you run your exposures. SO, you want short exposures.
How do you get shorter exposures? You up the gain, or ISO. Go for as high as you can reasonably go and still get at least some usable dynamic range. (And if noise is a problem at high ISO/gain, then back off.) And remember, while it is best to avoid shot noise, it can be dealt with also in processing. So, maybe accept more during acquisition, and work on it harder in processing.
How short should your exposures be? As short as you can get them and still rise above the background noise. How long? As long as you can go without showing field rotation. Try ten seconds. Then twenty. And continue as long as when you look in the corners of the frame, you do not see objectionable trailing or eccentricity. And remember, when you change target position, you change the rate of field rotation, so what is good for one celestial target is not good for another. And remember, as you are looking at them on your computer during the session, they will all appear dark. YOu must apply a screen stretch to see what you are really getting.
How many exposures? As many as you can get in an hour or two.
May I suggest another tack? EAA. Using SharpCap (which is free--but I do not know if it will work with your camera) you can monitor how all this is going in real time. All the stuff I said about field rotation and exposure time still applies. But, you are watching the image build as you go. You may find it much more productive than the traditional way we do things in general in deep sky imaging. And, if you save all of the frames, not just the one SharpCap is stacking, you can go back and apply all the tricks that the traditional "deep space" imagers use.