Yossarian,
Ah! Now I understand...
In that case, if you like the sound of Sennehiser, then it can be more straight forward. I think that the "Sennheiser sound" is very logical and progressive as you move up the line. I went from a 500 series, to a 600, to the 800 series over the years. Each step was better - clearer, better sound. Each level was a very good headphone for the money and shared a lot of the same qualities.
I would suggest to look around for the model you can afford now and purchase them. Then, move up over time. What I also like about Sennheiser headphones is that they retain their value as a used item and they are well known - so, they are easy to sell.
Best of luck, CHRIS