As to aperture, 200 mm will give you good results, but most observers seem to graduate to larger apertures. 250 mm should answer most needs, and unless you are getting excellent seeing conditions consistently (or become expert at processing) you probably will not find much advantage in going to 300 mm or more. Beyond that size, transportability will be a factor. However, for a fixed scope the focal length will be longer and longer with a larger aperture. I presume you are going for an SCT but I have had excellent reports from users of Maksutovs of around 250 mm aperture.
I would suggest that you think carefully about what focal length you will need to get a decent size of image. I use a 16 inch Dall Kirkham with a focal length of 400 inches. This enables one to get a decent size for Mars even at an aphelion opposition on the chip, without a Barlow lens, but I have actually found the image size somewhat too large for Jupiter near Opposition. I am mainly a visual observer so it does not matter too much to me. But as a Section Director I find the biggest complaint from (or obstacle for) observers is getting a large enough image. Some observers have coupled two x2 Barlow lenses together but this can lead to internal reflections and ghosting. There is a x3 Barlow on the market and that may be a better solution. If you intend to image Venus with a Barlow you should be aware that ultraviolet will be absorbed by glass and that a special Barlow will be needed, if you need a longer focal length.
Many observers have missed a discovery by taking too long to process their images, and I know several observers who always work upon them immediately after the observing run, and then send them in. So another piece of advice is not to accumulate your unprocessed images. You can always submit a better processing at your leisure, but report the result on the same day if you can. I find that some people can submit images six months late, but by that time their usefulness is much reduced.
I will leave others to give advice about manufacturers…..