James brakes are not really a problem. They are operated hydraulically by Lockhead & lot easier to set up than Girling in Land Rovers. The handbrake acts directly by a linkage on each cylinder, not a transmission brake like on a Land Rover. You won't get new slave cylinders, you will have to rebuild them. They are a bit fiddly but the cup is the same size as for a 101. Master cylinders are fairly easy to get & service kits.
But at least the clutch mechanism needs no hydraulics, it is just a pedal & linkage to the clutch.
As for tyres unless the tyres are good it immediately knocks £1,000 off the value in my view. For that is the price you might expect to pay if you found some surplus ones.There seem to be no sources of run flat tyres of the required size any more. So you have to rely on a chance purchase of someone having a clear out, giving up etc. Someone has fitted truck tyres but they are not RF so carrying a spare with a Pig doesn't really look right.
What you have to get used to is that there is no Humber spare parts autofactors where you can buy something you need. The B60 engine spares are available at a price. You can get brake hoses made up & there were a few servos around at a price. But most other things you can sort out equivalents or make up yourself.