I'm planning an interrail, too. Although mine isn't exactly like yours, I plan to spend one month. My plan is:
I've used the following method to plan the trip:
I've stipullated about two days in each city (trips at night as much as possible and in train). I think your plan has a weakness: the cities don't follow a continuous line. You could save much of the time you would waste on trip if you reconsider your trip and use it visiting the cities, which is the most important and funiest. Why don't you try:
you can have a good perspective making a simulation of the trip visiting rail.shop/bahn
: you can use it for any connection.
As for money I have no idea about how much you need. I know a guy who spent about 1000€. I think a good estimate is to sum the ticket price, the hostels/campaing area fees (about 25€ for each night) and a budget for each day multiplying by the total number of days (that guy used 18€, i plan to use 30€). Good luck! and sory for the big reply.