Hi folks

I inherited an old Brompton folding bike from my father - this is from about 1997 though he didn't keep any of the paperwork.
It predates serial numbers and the only major problem is he had it stamped with his surname and postcode on the underside of the crank housing. From what I understand no paperwork reduces any resale value and the stamp makes it almost unsalable.
It looks to be 3 speed with I think M-type bars, rear mudguard & rack and maybe a front mount for a bag - however, I'm still a novice with folding bikes.
It cleaned up Ok but I think it needs some work (at least one spoke is broken).

What I'm trying to decide is if its worth upgrading to 6 speed and possibly updating to disk brakes, or selling it. It's been in my shed for 3 years and used once..
It's red and I'm not keen on that colour..
But it's a Brompton which I've wanted for a while.

Quandry.

Well, being an old Brompton, and your father’s old Brompton are 2 different things entirely. IMO

After looking further with Brompton, it turns out it was built in 1991.

Why would this make it unsellable? With bike of more than 20 years possibly nobody expects paperwork like user manual, original reiceipt or things like that (though the existence would possibly be a plus but probably not lead to a massively higher price with a "ordinary" late 90ies Brompton).

With a Brompton of that age a lot of things are worth checking and upgrading as the Brompton has developed massively over time. Incrementally, but it adds up. When upgrading too many things you end up more expensive than with selling yours and buying a more modern used Brompton. And with bad luck and full upgrades to today's standards even higher than a brand new one. Mainly because Bromptons keep their price and therefor especially older models like your's seem to be overpriced on the market in comparison to newer ones.

Upgrading a Mk2 like your's to 6 speed requires a new rear triangle or massive tinkering as it lacks the mounting point for the derailleur. You can go for the 5-speed S/A SRF5w instead.
Disk brakes require a new rear frame and new forks, both from 3rd party like Ben Cooper. I would not invest that on a old short frame but only on a bike with the newer hinges and the longer frame. If such a mod is necessary is a clear no, though it is fun. But expensive fun.

So you have had a Brompton for 3 years now but only used it once despite claiming you've wanted one for a while. A bit strange. So before further investing in a new bike I would bang around a little bit with the one that you have to find out if you really want one and would make use of it.

Thanks for the comments, they're very useful.

Not riding it has been a part of severe depression I've had for the last couple of years.

The stamp would give me pause if I was buying second hand and after reading (perhaps too much) about the second hand market it makes me overthink on resale value. I have no idea, though, thus the queries.

Time to try it out!

Get the spoke fixed and the bike tuned up. It will give you a much better chance to enjoy it.

As a second-hand buyer I wouldn't let the stamp put me off, it's just adds to the history/lineage of this almost 30 year-old bike.

But, as the owner (and I assume namesake) it would make for me, a much more compelling reason to use it, and enjoy it for what it is, as well as for what it isnt (as in it isn't NEW, it isn't 6 speeds, and it isn't equipped with disk-brakes)

I would take Schwinnsta's advice very succinct advice, above if it were me.

give a go to a bikeshop to fix it up, greease the hub, fix the spoke.
also, one very good tuneup for this aged brompton is to retrofit a rear frame locking clip.
It keeps the rear frame to fold alone while uplifted and was very major upgrades in brompton after 2006.

Yes, thanks, the suggestions have been what I hoped - now I'm looking forward to trying it.

I would take to a Brompton dealer and have a full tune up done. Then ride it for a while than make your discussion whether to keep it or not. A Brompton in full working order will be much easier to sell and at a higher price anyway. Riding my bicycle has got me over my bad depression so much so I no longer suffer from it.

I'd keep the old one to ride around. It's not a race bike so no major performance benefit with an old vs new one if you ignore gearing. If you're not climbing big hills or have to speed along maybe 3 speeds is all you need anyway. I have a 2 speed and like it but I live in a flat area.