An Olympic barbell is 20kg for men's competitions both according to the IWF and IPF standards. That's what the manufacturers build towards. The other standard sizes would be 15kg for women's competition and 10kg for junior's competitions. Beyond that, standard plate sizes from 2.5k and up are:
50, 25, 20, 15, 10, 5, 2.5
From 2.5k and below, there are two options. IWF for weightlifting uses the 1kg rule plates, and IPF for power lifting (and most gyms) use the 1.25kg "biscuit" as the smallest plate.
IWF: 2.5, 2, 1.5, 1, .5
IPF: 2.5, 1.25
As to the sizes of the plates, the maximum disc (European spelling) size is 450mm or ~17". The 20kg plates reach that size, and the 25kg and 50kg plates make up for being heavy by their thickness.
If you fancy doing Olympic lifting (Clean & Jerk, and Snatch), I recommend getting bumper plates for 10kg and up. They are more expensive, but are made to handle being dropped. All bumper plates are the full 450mm diameter, and vary in thickness.
If you are going to stick with bodybuilding or power lifting style workouts, stick with the less expensive metal plates.
Personal recommendation: I have both sets of plates, and honestly, just using the 1.25kg plates as the smallest is just fine. Unless you are also trying to outfit Olympic plate sized dumbbells, I would recommend filling out the rest of the bar with 25kg plates.
So for your purposes it would be: (25 + 20 + 15 + 10 + 5 + 2.50 + 1.25) * 2 + 20 = 177.5 kg
That leaves 62.5kg to fill in. If you get a second pair of 25kg plates you'll have 227.5kg, and a third pair of 25kg plates and you can lift anything up to 277.5kg. Traditionally, you simply keep adding 25kg plates as needed.
If you are getting a pair of Olympic plate holding DB handles, then my recommendation will change. In that case I would get another set of 5kg and below plates so you can load them up the same, and then fill in the rest of the weight with 5kg plates. Anything larger than 5kg on the dumbbell handle makes it difficult to handle properly.
