Hmm. hadn't considered just running 3 gears .. simplicity is always a good idea I suppose.
My reasoning for having a dynamo-hub is that I aim to cycle to the train in the morning, then ride from the station to work (total around 2 miles) then in the evening either ride back via the train, or just ride home (around 7 miles). I think that having an always available, never running out, lighting source will encourage me to ride all the way home more often, rather than getting the train.
People seem very divided on the rear carrier, I rather think I want one, but it is around £100 which is pretty steep I suppose; but then nothing is cheap in Brompton-land

Can they easily be retro-fitted if I decide against one ?
The bike I test-rode had the M-type handlebars - there were others with the S-type and P-type. I sat on a few and they all seemed OK - are there any drawbacks between one type or another, or is it just aesthetics and user-preference ?
I worry about the tyres, I know that small wheels are harder to change tyres than large wheels and that Schwalbe Marathon are bl**dy hard to get off at the best of times - is this an issue - should I go for the standard tyres ?
Retrofitting a rack is a right fiddle, so probably the worst of all options.
I never really got properly organised with mine, but as
@Sara_H says, they can be made to work, so you may as well have one.
Marathon tyres are the best bet, you don't really want a puncture on a Brommy.
Removing the rear wheel is a fiddle, it's easy to lose track of how the gear bits go back together.
Removing the front is simpler, although you would still have to disconnect the dynamo.
Shimano use a cheap two-pin connector which is quite stiff and fiddly.
I have one on another bike and couldn't get the ruddy thing off with cold hands.
No quick release on a Brommy, so you would need to carry a spanner.
The other option, of course, is to patch the tube in situ,
I've never tried that, but it should be possible.
I've had two punctures, despite running Marathons.
I limped home on one, and took the bike in a taxi to a nearby Brommie dealer for the other.
The supplied pump isn't up to much, I think a lot of owners carry a decent one.
Going back to dynos for a moment, I believe the ridiculously expensive SON one might have a clever induction connection which means there's no wire to disconnect.
Worth checking if you decide to hang the expense and go for the ultimate Brompton.
There's some merit in that, it really is a bike you only need to buy once, so you may as well have it right.