Are you sure your caliper is seized?
How worn out are your pads? I had a front caliper binding last week, turns out that the sliders had a bit of brake deposit at the ends and because the pad was around 70% worn it started to reach that area of the sliders were the deposits were.
Disassembled the caliper, cleaned the pins, greased them, installed new pads and problem was resolved.
Check this before buying a new caliper. Alternatevely you can refurb yours for 45 quid.