The Terrano, having independent front suspension, means dropping the front diff and shafts, skid plates etc before you can get the sump off, hence the high labour. Your biggest worry will be just how bad the rust is. Chemical metal et al aren;t bad for small pin hole repairs, but I wouldn't use it for anything bigger and the surface will need to be sanded back to clean metal before it will stick properly.
If it were mine I would pull the pan aka sump and replace it. 4 hours is probably not even close to what it will actually take, but a days worth of work solves the problem permanently. If not I would at least try to spot weld it with a mig. Other wise braze it once out, but replacement is the real solution.
