Connective だって and use of せっかく

Question

It's hard to tell where the following breaks up as it's in one bubble.

まあまあ
せっかくだから[買]{か}いに[行]{い}ったらどうだ
[山田]{やまだ}だって[必要]{ひつよう}だろ

Anyway. My attempt at (somewhat literal) translation here is:

Now now,
Because it's troubling, how about if we went to the store?
It is necessary, 山田 having existed.

The last part makes no sense.

I'm not sure what the connective だって or the せっかく are doing here. Or the last だろ, for that matter.

EDIT

A new translation based off information provided in the comments:

Because it would be a shame not to, how about about if we went to the store? I guess 山田 needs to as well.

The character is responding to one a question from another character who is wondering if everyone has a certain item.

P.S. I've avoided mentioning the source because of the absolutely humiliating nature of the translation assignment (the same one as my classmate's from other posts, it turns out). I'll give a link to on one image, but caution you against reading the series. Unless you have morbid tastes.

http://mangamatome.tv/view_9995_3.html

Answer 1

My translation would be:

Well now,

since we've come all this way, how about going to buy one (bathing suit)?

Anyway, Yamada needs one, right?

Without reading the entire comic in your link, I am guessing that getting to the beach was kind of a journey. In this case せっかくだから means something like "since we've already done this difficult thing" or "since we traveled all the way here", etc. だって with someone's name can serve to call attention to that person, pointing them out amongst others to remark about something concerning them that may be unique or special -- in this case, the fact that he has no bathing suit and that the previous statement directly applies to him.