'70s metal/proto-metal

UFO - Flying
Lucifers Friend - Lucifers Friend
Flower Travellin' Band - Satori
Budgie - Never Turn Your Back on a Friend
Legend - From The Fjords
Sir Lord Baltimore - Kingdom Come
Deep Purple - In Rock
Rush - 2112
 
If you're going for Led Zeppelin, I reccomend getting How The West Was Won. It'll give you a really good overview of Zep's material and their influence on metal. Whatever you do, don't go for any of those best ofs though, they're obsolete. Every Zep song gets heavy radio play so the only valid "Greatest Hits" would be the Complete Studio Recordings. All the Zep compilations are just a random collection of songs thrown together in a random order.

I don't think Blue Cheer was as important as people say. Their early material is no more metallic than Led Zeppelin's first album, it's the same kind of hardcore blues rock. But if I remember correctly, their 1984 album The Beast Is Back is totally awesome and pretty metallic.

Some pretty cool non-essentials:

Captain Beyond - Captain Beyond
Bloodrock - Bloodrock, Bloodrock 2

You should also go for some Robin Trower because they are cool. You could go for Bridge of Sighs but that's cliche' so I reccomend For Earth Below/Live.
 
High Tide - Sea Shanties


I'll also second the following:

Budgie
Deep Purple
Rainbow
Blue Oyster Cult
Blue Cheer
Hawkwind
Bloodrock