Taking it one by one.
The SSL front suspension is adjustable for the spring loading and also progressive. It also allows a degree of adjustment for ride height when it is being fitted. The important thing is that the static spring load must be such that the "rebound" springs are not loaded (just free to turn by hand). The result is somewhat lighter steering because the turning stub axle is not under pressure.
This result can be achieved in other ways but the SSL system makes it easy.
Dampers that are adjustable give the possibility of them being set incorrectly. A good pair of non adjustable dampers designed specifically for the car is fine. If you are technically minded adjustables may be for you. Good dampers are important and the factory fitted ones, as with most cars, are rubbish.
Brake reaction bars are fitted to the front end and are a must. Logically they should have been a factory standard fitment.
The Panhard rod at the rear end is fitted laterally across the car and is intended to stop the rear axle moving sideways. It is essential on cars fitted with coil springs that have a rigid rear axle (independant rear suspension requires a different setup). The Morgan, however, has leaf springs that give a certain amount of lateral stability but with a Roadster the Panhard rod is a must, particularly if you occasionally do a bit of spirited cornering

If I recall correctly anti-tramp bars were fitted to the Roadster as standard from a certain point. If not fitted I imagine most owners would recommend the fitting. It would not be expensive but it would be worth considering.
Of course if the SSL5 link rear suspension is fitted the above is irrelevant

I agree about the speakers. On the Plus 4 the wiring for the rear speakers is found tied up behind the tool tray (once removed). They were used on the 4 seater version. I imagine the Roadster is the same. I have not fitted rear speakers because the rear area is almost always filled with luggage when we are on a trip. I have considered fitting small speakers in the panels just behind the doors but it would require a certain amount of dismantling to do so and my dealer does not want to do it. I am not sure that I want to do it myself.