Planning to fit a 5-Speed IGH: what Brompton to choose?

After fifteen years of mainly using the TfL hire cycles, a recent move outside the scheme's boundary has resulted in my trying the Brompton hire bikes for a few weeks, and enjoying them so much I now want to get one of my own.

The hire bikes are all slightly different, which has actually been helpful in understanding what I would like to buy: one had only two speeds, one had a larger front chainring, each has had a different kind of shifter, handgrips, and bell.

The three-speed bikes (at least except for one with a too-large chain ring) seem perfect for my commute—the Brompton gearing is definitely has a more useful set of gear ratios than the Santander Cycles do, with their mostly-useless 2nd gear—so I was therefore gutted to discover that the hire configuration (C-line 3 speed with dynamo lights and telescoping seatpost) isn't actually one Brompton makes any more.

Maybe that's for the best, though, because my commute is pretty flat and so I'd probably be wise to get a model with at least one more lower gear so I don't feel the need to avoid hills.

Researching the different gearing options lead me to the 2Bikes4Adventure's amazing rabbit hole series of videos about gearing (1, 2, 3, 4), and eventually I reached the same conclusion as /u/ConcentrateOk4702: the old five-speed internally-geared hub was the GOAT. I'd always thought the six-speed hybrid arrangement seemed awfully kludgy, and now it's clear to me why: the BSR version was obviously introduced as a stopgap when Sturmey-Archer went bankrupt in 2000.

The six-speed BWR is still a kludge but at least the wider range of ratios makes it more justifiable for cycle touring—but in stop-and-go traffic in the city—and I do stop, and then like to off the line quickly—it looks like I'd be constantly shifting 2nd—3rd—4th (or 1+ / 2- / 2+, if you prefer), making the presence of a derailleur maximally annoying. (At least downshifting from 2+ to 1+ can still be done while stationary, though, so that's something.)

The arguments against buying a 25-year old bike just to get one with a five-speed hub seem sound, though, so it seems like the Sturmey Archer SRF5 is probably ultimately the way to go, at least until Brompton deigns to bring the eigtht-speed Alfine hub to the C-line.

I'm not going to immediately mod my new bike (in part because Brilliant Bikes seems to be out of stock on prebuilt wheels and I don't fancy building my own), but I'd like to make sure the five-speed hub is an option.

Am I correct in thinking that I should therefore try to track down a three-speed C-line, in order to simplify fitting a five-speed hub in future? Or can the derailleur on the six-speed be somehow be used with a five-speed hub, giving a ten-speed bike overall—but with the emphasis on primarily shifting with the hub, rather than primarily shifting with the derailleur (as it would be with the new 12-speed models)?

Or am I just overthinking this, and should just try the six-speed, and will then discover that modern derailleurs are not as terrible as I remember from the two decades I had to suffer them before discovering the joys of IGHs?