Yeah, that ticket also sounds good, the only negative point is that it must be used on five consecutive days.... otherwise its good!
The examples of how to get to Sapporo in 2 days are for the Seishun 18 kippu as well! Only example 4 is for the other pass.
For you individual plan you can use
Hyperdia | Timetable and Route Search in Japan..
For the youth hostel: The one by the trainstation is quite nice and its good to meet people. i have posted a few pics in this
post (the first ones) of the place i stayed in Sapporo. It is new and kind of a capsule hotel but with a really nice spa.The good thing was that it was right in the city center (susukino). It cost 2900yen and the place is called Avinel, here a
link to tripadvisor. You wont really meet other travelers here but its a nice place to crash, cheap and new. The spa is really good (japanese style)