Hi Kolya and welcome to IIS!
Astronomy is a bit male dominated but don't let that put you off.
The 8" dob is a great starter scope. As you mentioned some, namely the GSO solid tube, do not come with assembly instructions. Mine came with out of date ones that didn't relate to the particular scope I purchased. If you buy from Bintel, they rebrand the GSO dobs as Bintel ones but I am not sure if they include assembly instructions. I found that it wasn't hard to put together as I had seen a few assembled so I knew were I was heading.
I can recommend going to an observing night or star party before buying anything as if you can see some scopes in action and get an idea as to what views they provide, ou will be more informed when making a choice. You will find that usually observers are more than happy to hae others look through their scopes.
With size, eg 8" vs 10", it is really a mix of factors, price, aperture and portability being the main ones. Once dobs getup to 10" and bigger they are large instruments.
Malcolm