Planetary photography is easy with a platform or EQ mount, at least the drive part is easy. It's long exposure deep sky shots that puts the heavy requirements on the mount.

Unfortunately, nearly all the problems with building a platform apply to a standard EQ mount as well except the EQ mount has a way to guide in declination and has room for a much larger drive gear and bearings. The larger these are the easier it is to get the precision needed. Still it isn't the average home machinist's project. The precision needed in the bearing alignment alone is beyond all but the most experienced. This is why you'll see few deep sky astrophotos on home built mounts of any type other than those taken with very short focal lengths such as a camera lens. Though for that Orion's very inexpensive EQ-1 does very well and is likely cheaper than home made. But that's mainly for cameras only, not through the scope shots.

Probably the cheapest way to make an accurate home brew EQ mount is to use a sector arm drive. These can be quite accurate as the radius of the drive gear is measured in feet rather than inches. A friend of mine built a very good mount (weighed 200 lb however) for carrying his 12" f/5 scope using this system when in high school. Getting backlash out of the gears was his biggest problem.

In any case starting a lot smaller than a 12" scope will greatly reduce the problems you face. See the many great astrophotos posted to the astrophotography section for examples such as to beautiful 7 Sister's shots with 4" scopes of less than 600mm focal length. There's a heck of a lot to image at that scale that will keep you busy for years.

Off axis guiding (using the same scope) is usually the easiest to master. Trying to use a second scope for guiding has the added problem of flex. If one flexes even slightly and the other doesn't exactly match then your guiding is off. Using the same scope removes this stability problem! It doesn't matter if the guide star looks like a comet or not.

The only time I found a second guide scope useful was when tracking a comet's motion. But then I used the main scope as the guider and the guide scope to take the image as it's much lower image scale was needed to capture the comet and its tail. But comets are so bright with a CCD today you can use short stacked CCD exposures tracking the stars, stack them aligning to the comet's core and get the same result. Stacking programs will do much of the work for you.

Join a local club, there are usually a few telescope makers in one that can give you the knowledge you need.

Rick