So my computer just got rebuilt, and I was thinking about getting the package of all 5 main ES titles.
(Arena, Daggerfall, Morrowind, Oblivion, Skyrim).
I've only played very little of Morrowind but a lot of Skyrim.
And I'm just wondering, the games are obviously getting old, but is this a good plan?
Are the older games (specifically the 1st 2) still playable and enjoyable? Even after playing Skyrim so much?

I've heard so many good things about Daggerfall and Oblivion, and I never got to fully experience Morrowind. So should I do this?

depends on if you can deal with the graphics and antiquated game play mechanics.

Well I know I can deal with Morrowind Oblivion and Skyrim, and those three games plus their DLC are worth more than $80, so even if I can't handle the prehistoric Arena and Daggerfall I'll still get a good deal right?

I think you've answered your own question.