Nagoya-Shiragawa round day trip

Konichiwa :)
1.I will be visiting Nagoya for a week and was planning on doing a day trips to Shirakawa village as well as Takayama and Kiso Valley from there.
I will be getting a Seishun 18 pass for these trips.
Although I saw on hyperdia.com that it will be impossible to arrive to Shirakawa early using just local trains...
I'm on a very tight budget so I can't buy any tour packages or going by Shinkansen..
Is there any cheap option to get there beside Seishun?
2.Also do I need to book a tickets for Seishun pass in advance or can it be done at the same day of travel?
Thank you in advance:)

Perhaps your HyperDia search to Shirakawa failed, because there is no JR train or bus that goes to Shirakawa. Seishun18 tickets are valid on JR lines only.

You can go to Takayama from Nagoya with Seishun18 ticket. For example, take the 12:30 train to Gifu (18 min ride), then take the 13:15 train to Takayama (195 min ride!). But the total travel time is about 4 hours one way, so you better think about staying overnight in Takayama.

I'm on a very tight budget so I can't buy any tour packages or going by Shinkansen..

As OGG3 noted, there is no train station in Shirakawago, and further there are no Shinkansen stations in Kiso valley.

Alternatively you can use bus to go to Shirakawago and Takayama. Shoryudo bus pass 7500 yen for consecutive 3 days is a big bargain.

http://www.mwt.co.jp/shoryudo/index_en.php
http://www.mwt.co.jp/shoryudo/timetable.pdf

If you have enough time to go to even Matsumoto, Nagano in addition to Magome, their Matsumoto-Magome-Komagane course 7000 yen for consecutive 3 days might be a good value for money.

See below for ways to access Shirakawa-go:
http://www.japan-guide.com/e/e5953.html?aFROM=2155_5950