I was fortunate enough that with careful adjustment of the saddle clamp, I get the right height with the standard seatpost at full extension. I believe there's a widget now for automagically getting the right extension every time *googles* "Saddle Height Insert" appears to be the thing, but if more than one person is using the bike, you're probably better off marking the seatpost.
Resist the temptation to over-torque the seatpost clamp. If you have problems with it slipping, clean the post with alcohol.
The Brompton bell is there to make a satisfying PING! as the handlebars drop down when you finish the fold. All other functions are secondary.
If you ever do find the need for luggage, the front luggage block system is well worth it. The bike handles well with some weight on the front, and it's quick and convenient. OTOH, as a telescopic seatpost user, saddlebags may also be practical without making the fold significantly worse.
The best way to do clipless pedals on a Brompton is with one of the quick release systems (there's no such thing as a folding SPD pedal, but a quick release one can be removed from the left crank and stuck in a bag, and as a bonus you can easily swap to flat pedals for use with civvy shoes): Either the MKS EZY system, or Wellgo QRD. AIUI the Wellgo pedals have better compatibility with Shimano cleats, but MKS is more popular.
Koolstop Salmon brake blocks are a worthwhile upgrade, if only on the grounds of rim wear. Regardless, Brompton braking is best described as 'adequate', with speed primarily regulated through the inefficiency of the bike as a whole.
Due to the fold, Brompton cable outers are extremely sensitive to length. If you replace the cables, measure carefully.
Ergon GP2s are a good upgrade to the S-type for an alternative hand position without compromising the fold.
Squeaks are usually the suspension block.
The rear end of a Brompton is a complicated system for collecting road crud.
Keep an eye on chain wear, my Brompton seems to wear chains a lot faster than my other bikes.