Hi Justme,
Congratulations on going ahead with the 8" dob - you won't be sorry and you will be in for some truly breathtaking views.
Everything Dave said is spot on!
Learn to use and familiarise yourself with the scope first. After a bit of time, you will be in a better positon to really know what you want. Beginners can often get caught up in over magnifying objects with a barlow and lose quality without realising it (a mistake I made myself - barlow was the first thing I bought). The seeing conditions will dictate when to magnify. In both my 6 & 10 inch dobs, I've found that a 7mm eyepiece hits the sweet spot nicely for planets. That being said, the 10mm that you get with the scope won't disappoint either. A 5mm can be a bit temperamental depending on the seeing, but is still good to have when the seeing dictates.
If you want to spend some money right now, let me take you shopping
... this easily adjustable observing chair from Bintel for $179 makes viewing soooo comfortable. I have one and won't part with it for quids.
https://www.bintelshop.com.au/Images/Stock/7201.jpg
Have fun!
Kind regards,
Suzy.