Improving O/R capability, The Disco 2 is a fairly capable off-road vehicle as it is, but if it's a purpose built mud-plugger you're wanting then a Disco 2 Metropolis wouldn't be where most people would start from.
Road performance and fuel consumption, They really go together. The first thing to consider is that you've got a 2 1/2 ton steel box with all the aerodynamics of a concrete block and powered by a diesel engine. F1 it ain't, neither is it a miser with the fuel. Consumption will depend on how you drive it, expect to upset your bank manager if you start driving it like you stole it. You can improve the driveablity by getting the engine properly remapped. Removing the EGR, possibly washing out the intercooler and maybe replacing the inlet hoses could help with performance, but don't expect miracles. Do NOT even bother to try one of those bolt on, stick in, super-duper perfomance increasing tuning boxes, all they increase is the speed that money flows out of your pocket. They don't work, they are snake-oil.
Comfort, It's a Metropolis FFS, it's the top of the range! Unless the driver's seat springing has collapsed, it does happen, then there's little you can do to improve things, other than considering some nice blingy rally type seats. The suspension should be like driving on air, largely because you are and the cornering sway will be under control of the ACE.
Even just preventative precautions for in future. As others have already said, regular maintenance, including all fluids and filters is important, as is getting the underside steam cleaned, checked out, repaired where necessary and then protected with Dinitrol or Bilt Hamber products. check and re-apply as necessary every three to five years.
As
@MJI has recommended, get the brake lines replaced with cunifer, it's even a job you could do yourself. The metal pipes used on the ACE system can and do rot, replacing them with hydraulic hoses is a good idea for the future life of the vehicle.
As
@neilly has said, make sure that you have all of the security information that you should, that includes the EKA code, radio code, key identity number and also a working second fob. Make sure that you change the CR2032 battery in the fob every two years or so, don't wait until they fail, they're cheap enough. A Nanocom , Hawkeye or Lynx diagnostic code reader is a good idea too, especially if you intend to try to look after the Disco yourself. Don't get fooled by the online adverts for things like the iCar code reader, the only code readers for DIY that are known to work properly with the diesel Disco 2 are the three named above. Download a copy of RAVE which you'll find invaluable.
As much as I'd love a really mean looking Disco. As I and others have said, a Metropolis isn't really the right vehicle to start with, too many electronic circuits to go wrong when you get it wet. Your better bet would have been a 300Tdi Disco 1 with a Camel Trophy cut and steel bumpers as a starter vehicle.
The correct selection of tyres will help in both on and off road performance. Talk to a proper tyre specialist for ideas, depending on your expected use.
