Maybe get a set of combination ring/open-enders going from 3/16ths WW (1/4BSF) to 7/16WW (1/2BSF) for starters, and perhaps a small set of 3/8" drive sockets covering the same sizes. King Dick and others do neat sockets of excellent quality, not ridiculously expensive. IMHO far more useful than 1/2" drive, 90% of the time.
These would cover most of the mechanical bits. Dimensions of hexes and shanks are the same for BSF and BSC, the shanks are different for BSW but the heads are the same pattern.
For rear axle nuts if bigger than 1/2BSF, and also for the oil tank big filter nut (which is, and is a regular thing to visit!), best get those sizes individually unless you're OK with using adjustables (not!) - or a metric / AF one's a close fit. Steering head adjustment ditto - not actually sure what the dimensions are on them, just that they can be hard to get onto properly sometimes.
For fiddling with the points and 'leccy bits you'll need BA spanners - with the slenderest possible jaws as there's norralot of space. A set of 'Terry' spanners is a cracking little investment if you can find one (there are plenty on e-bay as a rule).
Threads are as GB says. 1/4" to 3/8" would cover most needs if going for taps and dies. Tracy Tools has decent choices at decent prices (and plenty of people sell bolts and setscrews to make life easier on the die front).
Allen keys - a few imperial ones should do it - can't think there are many to deal with unless you've got 'em on the timing cover and the primary chaincase, or have SRM tappet adjusters fitted.
If thinking about 'special' tools, depending what clutch you have, might be worth grinding a notch in an old wood chisel or big scrooge to make doing up / undoing the spring cups easier.
A weapon to get onto the front brake backplate retainer nut will be useful, and the lack of is a complete PITA when you need something.
Another useful thing might be a 'pushrod comb' - unless you have very dainty digits. Can make refitting the rocker box quite a bit easier.
And if you have to take the primary side apart, you'll be into getting the clutch centre off (puller can be needed) and the ever-so-tight shock-absorber nut off the crank. But those are probably pleasures in store, along with taking off the pressure relief valve and one or two other quite big fasteners round the place!
The pain in the pocket shouldn't be too bad for most of this, but threading tackle can cost, as we know, if you want lots of it in decent HSS quality.
