Dave
I have a ST120 and bought it for public outreach events under dark skies in places like Northumberland. It excels at wide field views and is really easy to use. Mate it with an expensive wide field eyepiece and the effect on big objects like the Andromeda Galaxy is jaw-dropping. But here are few points to consider. If you want to look at planets and the moon a five inch mak would be better. In the synscan flavours you get tracking which makes viewing at higher magnifications much easier. Yes you will have to provide power and use a good dew shield/dew controller. Also the AZ3 is not a good match for the larger ST120 tube, especially if you start adding a heavier diagonal and eyepieces. Not only is the whole thing shaky but you will find it very difficult to adjust the altitude bolt to the right tension. Do yourself a favour and plump for the AZ4. I picked up a mint condition second hand one for just £80. The tube balances really well. One more suggestion. I also have a Celestron version 80mm ED scope (F7) - come up second hand for about £170 - also used with the AZ4. Very little false colour, good on solar system, but still wide field enough to get lovely views.
Rich