It is hard to tell from the pictures but I think that someone has had a go at adding to the seam welding where the top of the side member overlaps the main crossmember, and that is usually done on non-genuine frames where the spots have been skimped. The welding, if it is that, looks extremely poor and ineffective, but it may actually be dirt or seam sealer.
If I had it right in front of me I might be able to determine that it is not too bad, and just needs cleaning and painting, or preferably galvanising (which would kill the rust, for 50 years or more, otherwise it will remain in the seams), but from these photos I can only suspect that it would be a high risk purchase, best avoided. Running a Mini is NEVER CHEAP these days, and it is extremely inadvisable to skimp on safety critical parts. Sorry, but I have to recommend avoiding it.
A cheap new subframe will need some seam welding where I have described above, as certain unknown manufacturers (or maybe the ONLY manufacturer of non-genuine frames. Chinese or North-East England? Does anyone know?) skimp on the highly stressed spot welds to save a few pence. It may also need the strengthening gussets welded in behind the cone abutments. There are also may tales of alignment problems. It is therefore preferable to only use a genuine Heritage frame, and they are expensive. An accident caused by a subframe failure on the road is even more expensive.