When a card is graded "altered"
 

Is that the lowest form of authentic ?
Is say a 52 Mantle by SGC and determined altered below a one ?
I always wanted a 52 Mantle and am looking at one as the above. Is SGC reputable ?
What would I be buying ?
Thanks.

Depends on your purpose:
Are you talking about 1) expected sale price, 2) set registry value, 3) interest in the community?

An authentic card could be skinned (back removed), or damaged beyond comprehension, while an Altered card could have some recoloring/pressing or trimming and look NM-MT. If that's the case, the NM-MT looking ALTERED cards will likely sell around the price of an SGC 3.