Really glad to hear your brother is doing well now and this is a wonderful gesture from yourself. Whatever you buy him, he'll love I'm sure.
There have been several key watches throughout the years in terms of James Bond. The main culprits would be...
- Rolex 5513
- Omega Seamaster 2531
...now the above are in your budget but long since out of production. There must be a dealer somewhere with a NOS Seamaster in their window but finding it would be a chore. Therefore, finding second hand variants would be the only route with these.
So, if it was me, I would narrow it down to...
- Omega Seamaster Planet Ocean 8500 RRP £3950 (quite heavy but I got used to it and would have another any day)
- Omega Seamaster Aqua Terra 8500 blue dial RRP c. £3750 (smaller than most watches these days but very elegant)
Both of these were featured in Skyfall, with the AT being shown throughout most of the film.
The RRPs are of course out of range, however, I know of at least one dealer that will offer 20% or so discount in Manchester and also, Swiss watches direct on the internet; A grey dealer but you'd still have full manufacturer warranty. Even if you don't use them, you could use their prices as a bartering tool.
Hope this helps and let us know what you buy for him.