I'd break things down into parts:
1) What are the benefits to you (and anyone else who will use it) of having an erg at your home, instead of you just using the ones that are presumably available to you at your school?
2) What are the benefits of buying a C2 instead of another less expensive model?
Also, consider where it will be located - if you can't find a convenient place of it you won't be able to make the sale. (Unlike me, I doubt you'll be able to put yours in the middle of the living room.

)
Regarding facts, you could search E-bay "completed auctions" to get a feel for what used C2 rowers are going for - that should give you and your father an idea of how they hold their value over time.
Good Luck