Yes the TV amplifier is still available from the usual MG Rover parts suppliers the part number being:
XUC100771 Aerial Isolator.
Rimmer Brothers are selling it for £63.37
http://www.rimmerbros.co.uk/cgi-bin/...rtID=XUC100771
But the rear screen in your current car will not have the correct cables bonded to it that come from the in-built TV antenna, so even if you did get the aerial isolator, you wouldn't be able to connect it to the cars TV antenna.
Digital TV tuners are available, to replace the Video Module in the boot, you'd need to look for BMW compatible units.
JohnDotCom has plenty of info on which units are suitable.
The Steering wheel interface should plug into the wiring harness in the dashboard behind the TV screen, not in the boot - that could be the cause of your problems.
Here is a picture of where I believe it should go, but I'm not absolutely certain.
The instrument pack can be changed for a High Line unit, but you need to buy a brand new unused one, as it has to be coded to the rest of the car. It carries data specific to your car including mileage etc, and acts as a gateway to most of the cars electrical systems. Second hand ones cannot be adjusted to fit into new cars. So it's quite a costly excercise I'm affraid, as you need the car to be connected up to a T4 computer which MG Rover garages use to service your car.
In addition you need to replace the indicator stalk on the left side of the steering column with one that has the trip computer button selector on the tip. This is the only way to select and reset items on it. The TV screen only
displays a full page of values such as fuel consumption, range til next fill up, 2nd trip distance counter, and over-speed limit value.
A DVD player can be easily connected to your cars TV A/V input, all you need is an adaptor like this one
http://www.toysinyourcar.com/acatalog/rov_av.html