I think I can answer your question about Jupiter and Saturn in an 8" Dob.   The answer is Yes and No.  Yes, that image of Jupiter is not difficult to duplicate with a 5-8mm eyepiece in an 8" Dob on a night of reasonably steady "seeing".   And No, the image of Saturn is going to be difficult to duplicate at the eyepiece.......you can see it as clearly (maybe even slightly more clearly), but it will not appear that large.....unless the seeing is near perfect and you can use a 3mm eyepiece or a barlowed equivalent.   Jupiter's disk is quite a bit larger than Saturn's....especially now, with the rings all but disappeared.   I would go ahead and get the 8" Dob (or even a 10" for that matter).  You will not be disappointed in the views.   On nights of steady seeing, you will be amazed at the amount of detail that is visible.

Do be aware that on nights of poor 'seeing' conditions, the best telescopes produce poor views.  Some nights Saturn and Jupiter will be fuzz-balls with little detail.....other nights they will be so clear you will feel that you can reach out and touch them.  Seeing conditions make a HUGE difference!

Your Dob will probably come with two fairly decent Plossl eyepieces.....a 25mm, and a 9mm.  I would get a 2x barlow lens to increase your flexibility......turning your 25 into a 12.5, and your 9 into a 4.5.