Your local shop will have a very low cost replacement and can get everything installed quickly and easily. If you break a spoke you should get it replaced right away or replace the wheel right away. The minimal money you didn't save by not spending the $10 is now even more moot. Just take care of your bike and it can take care of you. Save money where it makes sense but being overly cheap for some reason that doesn't make sense when it ends up costing you more well...uhh...not sure on that one.

They do make 7 speed cassettes I would just stick with that. The spacing is generally the same for 7 and 8 speed but that 8 speed cassette is a rather small ratio and might not be what you want for that bike.