Any fork for that money will be a budget fork, ok if using on road, but will only last a short while if using off road, you need to be looking at the £150-200 for the start of anything decent for off road use.

For the mounts, IS is becoming obsolete for forks, with most manufactures now using the Post mount system, are you sure your calipers are not Post mouts, as most disc calipers are now Post mounted, with IS adaptors for older forks (5+ years old).

For the size, most forks will be 1 1/8th, you will find very few 1 inch forks, and if tapered or 1.5 it they will be higher end. also, if new, they should be un-cut, and will be longer than the 210mm you need