If you can weld and have the necessary kit, or if you have someone who can do it for you at reasonable hourly rates, your best bet could well be an M-Machine inner and outer sill sub-assembly. That cuts down the time to do the job.
There is almost no chance that it will just be an outer sill. Once you cut the outer off be prepared for the inner to be in bad shape as well.
From the photo it is noticeable that you also need a new door skin. That is not unusual either.
The issue I have is if i cut it open what else am I going to find, I know no one can say as they havent seen the car. The inner and outer sill would be a good option and I know someone who welds for 10 cans of John Smiths which I think is a good hourly rate.
I do not have access to a garage so what would you say for timescale, cutting out and welding new in? My biggest concern is opening a can of worms and not being about to get the car moved (in the progress of buying my first house, which unfortunately will not be complete by time MOT is due)
I want to get the this car back on the road, but I dont want to bodge it. At the same time Im worried I wouldnt have the facilities or funds to get it sorted if i do cut it open