I think Arelicat covered it.
I got an M6R on the B2W scheme last year. That's moustache handlebars, 6 speed and rack. It almost squeezed under the £1000 limit then I added SON lighting which went well over. BikeTrax were quite happy to oblige.
The front luggage makes them much more useful. It's about £15 for the block and the bags aren't cheap.
I prefer S bars, the flat kind, but they restrict the luggage options. You can't use the biggest bags or the folding basket. I find the basket works well on the M6R. I can fold it up with the basket in place and roll it round a shop putting things in the basket as I go. This saves having to carry another basket or getting a trolley. It also stops me getting too many messages. The basket will fit on an S but the brake cables foul it.
If you want it to roll nicely when folded then the stock rollers are best replaced.
Tyre options are Brompton's own or Schwalbe Kojak, Marathon or Marathon Plus. There will be a studded Marathon later in the year. Kojaks are bald and lovely but more prone to flats than the Marathon/+. I've not used Brompton's own tyres for years so I can't say much about them.
I do use the rear rack to carry stuff but other responses are correct. You can't fold it with something on there. I only use it when I can't put things on the front. It has bungies and wee tabs on the rack which stop them pushing things too far forward.
I'm not sure the Ti is available just now. I think the Russians might have something to do with it. I do have an S2L-X, that's a 2 speed super light with all the Ti options. I got that one for the extra va, va, voom. Other reasons are it's a bit lighter to carry up the stair and it doesn't rust. The rear triangle can suffer a bit from rust. It might be difficult to justify…
Considered opinion generally comes down in favour of either the 2 or 6 speed. The 2 is light and doesn't use an SA hub so it's a wee bit more efficient. Standard is a 56" and 74". The six speed doesn't add much weight over the three and gives a wider range with more gears. The three has the advantage of being a little simpler and there is less to go wrong but if you decide to change to a six then you need a new wheel and changer mech. It's easier to change a 2 to a 6 than a 3 to a 6.
Chain and sprocket wear is more of an issue than it is on cumbersomes. In a dirty winter I can wear out a chain in a month. In the spring/summer it's much better. The three speed sprocket can be flipped and reused. The 2 and 6 speed sprockets can't. Tyres are half the size so don't last as long but they're not half the price.
I've got a Brompton bag to put mine in when required. You can also use an IKEA Dimpa which is cheaper.
You're likely to find the steering feels twitchy at first. This feeling doesn't last for long. You might also find your heels striking the rollers on the rear triangle. Again this doesn't happen for long.