There is a true pure and what people call an offset pure where the attack and strength or mage or range doesn't affect the combat levels. So for an off set pure Range like what I have he is level 80 with 80 ranges with 70 defense and a high prayer not a pure right? Well it is because look the range is where all of my combat levels are coming from. I am a member, I want a high defense level knowing about all the goodies members get to fight with and now I can uses barrows armor. Enough RuneScape Gold is a big help on your adventure in RuneScape. The more, the better.
Another example of a off set pure level 99 range level 99 defense and level 52 prayer he would be about level just over 100 but now I can have give him 69 attack and strength without having attack and strength affecting the combat level at all. But now for people who don't see that as a long term goal to get to be one of the greats just want a low level pure PKer then I would say it like this.
Level 1 - 4 defense levels per pure level to keep combat down but keep your defense for a pure strength I would say to begin with 40 attack is a must to wield a rune weapon, for members look into 60 attack for dragon weapons. For a pure mage 2-5 defense levels per mage level, keeping in mind you might want to get a few attack and strength levels to help you get your defense up when you want to get it up.