Realistically, you're looking at a new front end here. To sort the scuttle & inner wing both wings have got to come off, at which point, for what they cost & the minimal extra work, you may as well do the A panels & the front panel at the same time, if only to ensure the rot is gone.
Whilst the repairs to your floor pans might be solid enough to satisfy the MOT inspector for now, the classic mini is pretty much an enthusiasts car these days, so the chances are that most prospective buyers will want to factor in the cost of repairing these permenantly/properly as well. You also seem to have rot at the front of the n/s door step & coupled with oversills & the state of the outer sides of the floor pans, you're probably looking at inner & outer sills + jacking points and at least one door step. The boot floor doesn't look too bad, but then the photo doesn't really show the problem areas very well. Corrosion to the boot floor tends to occur in the corners (particularly underneath the fuel tank) and further back toward boot hinge panel. There's clearly some corrosion to the o/s corner and the o/s rear wheel arch also looks a bit suspect, but it's hard to tell from the photo if it's significantly corroded, patched or just in shadow.
As for value, a standardish 80's 998 in need of this sort of work will typically go from around the £600 mark. As yours has a couple of months MOT on it, then you're looking at a bit more, simply because any prospective buyer won't need a trailer to drive it away. My advice would be to advertise it at £900 with offers. Advertising it the other/wrong side of the £1k mark is likely to put a lot of buyers off and there are better cars out there for that sort of money. Failing that stick it on eBay at a low start with no reserve & it will go for it's realistically worth.
If you want to minimise your losses, then I'd suggest you remove the stereo and accessories you've added and refit any original parts. The aftermarket parts that you include realy wont increase the value, but you might well be able to get some cash back for them if you were to sell them desperately.