For casual use, the Synta ST80 (Orion, Celestron, etc.) is totally
adequate. Optically, while you can tell the difference vs. a thousand
dollar scope, it's VASTLY better than a cheap spotter like a Bushnell
Spacemaster or Bausch and Lomb Discoverer.
The main drawback vs. "real" spotting scopes is fieldworthiness. It
probably won't be good to take it out in the rain, and might fog if you
are going in and out of a car in cold weather. It's not very rugged,
but it's cheap enough that perhaps you won't cry if it gets battered
after a while.
I bought a NexStar 80GT some years ago, and didn't much like the GOTO
mount, so I got some rings and put it on a video tripod.
I just use the 90° mirror diagonal for spotting. Unless you need to
read something, the L-R reversal is easily tolerated. A 45° diagonal
would be a bit more ergonomic, but either is better than
straight-through when shared by users of different heights, so I'd say
DON'T get a porro prism.
With its short focal length, plossl eyepieces short enough to give high
powers (i.e. <10 mm) have poor eye relief, but otherwise work OK. I
use my 9 mm Nagler (old version) a lot, which gives good results @ 44x.
A bit more power is sometimes desirable, but I don't want it enough to
buy a 7 mm Nagler just for this purpose. If I was starting over,
maybe, but 44x is often pretty good.
I've used 32 mm and 25 mm plossls quite happily. My SO recently gave
me a 24 mm Panoptic, which is very nice, but rather excessive.
Unfortunately, I didn't like the 17 Pan as much (weird field edge), but
it would give a more reasonable power. I'm now wishing I'd had a
chance to look at the new William Optic UWAN 16 mm, since that might be
a killer low power unit. But again, for casual use, an inexpensive ~
15 mm ep should be acceptable.
I compared the William Zenithstar 66 peztval side-by-side with a Nikon
ED spotter. They were pretty close, but I think the Nikon had the
edge. I didn't side-by-side the ST80 and the William, so I won't say
anything about their relative optical quality. I think the petzval is
cool, but since I don't image, I have no use for its supposedly very
flat focal surface. Ultimately, I decided that the Zenithstar gave up
too much aperature to the ST80 to make it worth having both. But it
has a much better focuser, and it is a lot prettier! So if you
consider form as well as function, it might be a good choice.