Brompton owners

dancing partner wants to buy me a brompton for my birthday (or course ti and raw laquer)
Good choice.

what is a sensible gear ratio to go for on a brompton, am thinking of going for the 6 spd option?

Depends on the gradients it'll be ridden on. For round town the 6 speed is probably overkill. The only time I used the lowest two gears was climbing up from South End Green to Whitstone Pond. The rest of the time the middle two were fine, and the top two were a luxury.

is it worth getting the son hub and lights?

Don't have them on either of mine - USB rechargeables are good enough for me.

can a rack be fitted at a later stage, and will it take my ortlieb pannier bags?

Yes, but as others have said, it's a PITA

are there better/lighter mudguards than bromptons own?

There's a nice man in Hong Kong does carbon fibre ones. They're not better than Brompton ones in use, but they are carbon fibre. Very not cheap though.

is there a shop that can sell me one without the handlebars or seatpost? long term would replace bars and seatpost with titanium but if i can get one with out the steel ones i'd be happy to do it that way

Doubt it. Seatpost from Brompfication and flat handlebars from J&L in Taiwan would be my recommendation for titaniumification.