Not all zoom is created equal
Firstly, the picture quality from the HV20 and HV30 is excellent for the money. Now on to the other issues.
There is no quality issue as such if you stick to optical zoom. Digital zoom just means that only a portion of the full-res image is recorded, with an effect on quality comparable to cropping a digital photograph and then blowing up the cropped portion to full size or beyond: what was hi-res to start with becomes lo-res pretty darn quickly. Contrary to what the writers of programmes like NCIS believe, you can't add new pixels to make up the difference in sharpess, you just end up with chunkier pixels, with a resultant decline in sharpness. Because you can achieve the same effect in the edit, there really is no justification for ever using digital zoom - unless you simply can't make out what your subject is doing without it. When your camera arrives, the second thing you should do is disable the digital zoom function in the menu. The first thing is charge the battery. Obviously.
Add-on telephoto adapters are a separate issue, though they're part of the overall equation if you're looking for a boost to the built-in zoom. Unless the quality of the optics in the adapter is appalling, telephoto adapters (or wideangle ones for that matter) are always going to result in sharper pictures than digital zoom. But, even aside from the fact that telephoto adapters are expensive, they usually result in a minor reduction in quality, because the camera was designed with the built-in optics in mind. The adapter is a compromise, and any compromise has a flow-on effect on picture quality, though it may not be especially noticeable. Sometimes it's just a matter of increasing a lens's tendency to flare in bright light.
If you decide to use an adapter, try to stick to the manufacturer's recommended one. But the usual rule applies: if you want a camera which can handle high-end applications like extreme telephoto, you either need to fork out wads of cash and buy a camera that takes changeable lenses, or compromise (in all of the senses of the word discussed above) and try an adapter.
But avoid digital zoom. Waste of time in my opinion. Like buying a new car and punching a hole in the exhaust to make it sound like it has more grunt.
The final point is that a 10x or 12x zoom is heaps. When I first got into photography, I could barely afford a 35-70mm zoom. That's a 2x zoom. But things have now "progressed" to the ludicrous point at which a zoom differential five times greater than that still isn't enough to market a consumer camcorder, apparently, and manufacturers have gone to the ridiculous length of providing 200x digital zooms! This would be laughable if it wasn't so pointless.
Most camcorders skimp at the wideangle end of the lens, not the telephoto end.