So far you've got 15 cities and one country (Belgium). You'll need to allow a full day to travel between cities (check out of hotel, get to station or airport, travel, find next hotel, settle in). So allowing 2 days per city plus travel time, that comes to 48 days.
Only you can estimate the cost of a trip like this. It depends very much on whether you travel 1st or 2nd class on the train, what type of accommodation you want - hostel, cheap hote, 5* hotel, whether you'll self-cater or eat in restaurants, entertainment, sightseeing costs etc.
Buy, or borrow from the library, some guidebooks to the countries you want to visit. These will give you an idea of the costs of accommodation at various levels, transport costs, food, and suggest itineraries. Also check out the cost of transport on relevant rail and plane websites.