You don't say how many miles the bike has done or whether the suspension has been replaced. If not the rear shock is probably goosed by now or soon will be. I would consider after market shocks. Wilbers are great from Revs racing in Birmingham. I would go for the slightly lower shocks ( not the lowest they do....the bike looks weird and you will never be able to sell it). Not only will you lower the bike a little but will also vastly improve ride and handling. When you come to sell the bike you will also get over half your money back if they remain in good nick.
On the seat, I wouldn't recommend a Sargent low seat. They are lower, but they are also wider than stock at the front, forcing your legs out making the bike hard to manage. Either go for a BMW low seat but the much cheaper option is to go to Tony Archer in Huddersfield as other have suggested. Your reprofiled stock seat will be built to your specifications and probably much comfiest than the torture device issued by BMW.
The shocks will set you back about £900, relatively straight forward to fit otherwise Revs will fit fo you, as will any bike mechanic.
Cheers