As Clive says, what are the door gaps like. If they are really good tack in a 25mm box section across the gap so the door still fits so you can double check before going ahead and properly welding if the gaps are bad you want a gap tool you can bolt in to hold the opening as is then once you take the sills off you can adjust the gap correctly by turning the nuts, here's one I knocked up but you can buy them, or maybe borrow one. I just paid £165 for the full floor, £10 middle sill then about £28 for the inner sill, I got lucky with a new outer sill and payed a tenner but i think they are normally about £45 oh and lower A post was £62, if your outer is good drill out the spot welds carefully to re use the sill, I would be inclined to do the full job as you can see the full extent