When I had my IUD removed, I was able to get pregnant immediately since it was not the hormonal IUD but rather the copper one. However, I was placed on birth control due to cysts on my ovaries...still got pregnant even though I had the cysts and was on birth control later on.
You would just need to ask your doctor based on the type of IUD, whether you can conceive immediately or what to expect. Sometimes birth control methods will take 1 week up to 3 months before allowing you the ability to conceive. But the hypothetical is once you do not have the protection, you are taking the chance of pregnancy.
As for the weight loss, since you are considering conceiving, I would sit down and speak with the doctor to see if you are at a good weight, overweight, or underweight. I found it amusing with WIC when they advised me that I could go up another 20 pounds...ive been overweight most of my life and just about fell out of the chair when they said "go ahead, its okay to gain weight". Ironically with this pregnancy, I have lost about 20 pounds, and while I am trying to ensure my child is getting everything she needs, I also am trying to avoid gaining any extra weight. With my other two, I gained approximately 20 pounds or so a piece.
If anything, just make sure you are intaking prenatals already, since you plan to conceive, a well balanced diet, and exercise. Dieting doesn't work since you end up denying foods you can have in moderation and you also tend to cut out essential vitamins, nutrients, and minerals. Best advice, just speak with your primary or your ob/gyn to see what they say.