You could also completely remove the existing shelf, and rebuild it a little higher, which would give you more width without intruding into the room any. Or do both, raise it a little to gain 15cm on the back, and come into the room 15cm, to give you around 65cm width.
Secondly, you can build your own tank for half of what that company quoted you. Talk to a glass company about buying the glass. Use the glass thickness calculator on this site to figure out how your cost increases as glass thickness does. Find your 'sweet spot' where your budget and a tank height you like meet. Order the glass and a case of 100% silicone caulk and go to work. For a tank that big, I think I might build a wood jig to help me hold the glass. (basically an L shaped piece of wood to hold the glass flush while the caulking dries. You might make a wooden 'frame for the bottom of the tank anyways, just to add some security.
Lastly, for me, here in America, the tank itself is usually about half of the cost of the complete set up. Between filters and pumps and decor and substrate and everything else. Then I usually spend about half the cost of the tank in stocking. This is a general rule of thumb for me when I am thinking about another tank.
For example, if I spent $500 on a tank, I'd probably spend around $500 on accessories (lights, pumps, filters, etc), and about $250 on fish to go in it.
