Cool, I have a friend who works in Town Planning, I'm guessing that is the same thing? It's pretty interesting stuff!
Your goal of 100kg sounds more than reasonable. Many of the problems encountered are when you are obese - being overweight can make medical problems worse, and can increase your risk slightly of conditions such as high blood pressure, diabetes, etc, but at your age, being a bit overweight would still be acceptable. Ideally, we'd all be at healthy weight, but that's not always possible!
Remember that the BMI tends to overestimate as well - especially for me - so if you have a lot of muscle your BMI will probably say you are overweight, when really you are healthy. Maybe when you get closer to goal you could weigh yourself and check your body fat percentage at the chemist and this will give you a better idea of where you're at.