Ice age campaign

The DMG itself has a bit of advice on low-magic settings. Generally speaking, it acknowledges that the D&D game is balanced on the assumption of magic existing. A monster or encounter CR is based on your PC party having access to magic equipment and abilities comparable for their level. That is to say, I can see why you might seek out another system. I would look for something related to Conan the Barbarian like Riddle of Steel, or in the wider world of low-magic settings:

Low-Fantasy Gaming

If you are sticking to 3.5, I would pick up the Frostburn and Races of Stone titles. They are chock full of ice-age setting goodness, with a lot of excellent weapons, gear, cultures, monsters, etc. like you would find in the ice age.
If you want an idea for a balanced experience that still uses 3.5 rules, check out Book of Exalted Deeds. It includes a 'Vow of Poverty' variant for characters living an ascetic life. The character swears off all possessions, including magical weapons and armor, so the VoP feat provides alternative bonuses to improve character survivability as they increase in level:
Exalted bonus to AC increases with level
Bonus feats
Free Endure Elements
'Magical' attacks like a Paladin
Deflection bonus
Resistance bonus
Ability Score boots
Natural armor bonus
Resistance to mind effects
Damage reduction
Energy resistances
Freedom of movement
Regeneration
True Seeing

In general, the way you make a low-magic setting is to just not provide the characters with magic items or services. You sort of have to build a world around them, but that should be part of the fun.