If the rot you speak of is in the external cill, it can be fixed with a half cill. If any of the stiles or top/bottom runners are rotted they can generally be repaired (pics of rot please) etc.etc.
Your window looks o.k. from the pic but some more detailed ones will tell more.
You asked whether the windows could be fitted from the inside however and my answer is still yes should you still decide on that course of action.
First you would have to remove all the reveals, plaster, wood - whatever, until the casing edges are completely revealed. Then remove the opening sashes, your now left with the casing intact, by cutting through both the bottom runner, cill and all, and the transom above, both at the angle,
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you can now crowbar the left hand side cill/runner up, allowing the left stile to be crow-bared off the wall at the bottom. Next use a hammer to smash the left side cut of the transom up and away, when all is aside you can crowbar the right casement stile off the wall, imploding the frame at he bottom.
If all's well the frame along with it's fixings, will be free of the wall except at the head, which will be just holding it in place. By wiggling and pulling the whole structure in and down (some crowbar force may be needed here) you should be able to free it from the slap (aperture). Some cleaning up of the slap is now required, and the new frame fitted.
That's a fairly abbreviation version of events, but should give you the gist of the work involved.
Let us know what you decide...pinenot
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