@Nutty_Death messaged me with a link to this bike on ebay - reposting it here if anyone else wants to advise (only thing I would add - it's very tidy so likely low mileage, but does seem high price for the age). Di2 is 6870 and although the listing includes a picture of an external battery charger...
I don't see one mounted on the frame. So unless it somehow has one tucked away somewhere I can't figure out, it has an internal battery with SD50 cables - probably in the seat tube??? If it is an external (can't see any wires out the frame) Edit:
Found it, mounted below the BB. It wouldn't take much to fit an internal battery. So I would factor in the cost of a battery (£120-130) and charger (£150) into your thinking as you don't know how reliable the charge/life of the existing might be and wanted internal
*(although no issue with running an external battery there by all accounts). Maybe a Wireless unit (£50-60) so you can keep an eye on it and get alerts on a head unit, also can then play with etube and stuff.
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Looks like the RRP was £4500 but they sold at around £3000 when new - https://www.sigmasports.com/item/Trek/Domane-59-C-Ultegra-DI2-Road-Bicycle-2016/98NW
Also being a 60cm you're after big frames, so towards the top of sizing range will see more deals/more stocks around new, would we expect that to mean S/H value drops more so depreciation is higher when compared to more medium sizing, I don't know. Maybe.
What heart rate monitor do you all wear?
I've been using a Wahoo Tickr the last 18 months or so and I'm pretty sure it's about to kick the bucket. I may just need another strap but it seems they're known to not be the most resilient.
Any others worth checking out? Or will any Ant+ do the same thing?
Had a TiCKRv1 (Blue) for 6-7 years before it eventually died. Almost bombproof! Bought an amazon strap as the original almost rotted away!
Replaced with a TiCKRv2 (Black) failed around 18 months old (visible moisture ingress), was prepared to buy another and messaged Wahoo to ask advice on how to mitigate (the v1 had a rubber seal around the battery chamber, the v2 doesn't), a couple of emails in 2-3 hours and they'd sent me a new one which arrived next day. Amazing service & unexpected support, so even out of warranty and with a bit of a design flaw they stand by keeping their customers very happy!
Still consider if this v2 fails and I'll happily buy a new one - as I've got good loyalty from the v1 and the way they've handled things with this v2 so have no need to look elsewhere. Also consider 3-4 years of use (with 1 free replacement when needed) for a generally otherwise reliable £40 product is a 'known' spend. Like something consumable. £1 a month for HRM!
Before the first TiCKR I had a Garmin, think it came in a deal with a head unit (Edge 500?). I had 2 maybe 3 replacement straps in a couple of years with it but was never that happy with it's reliability (or lack of). TiCKRv1 was a cheap alternative and my first foray into Wahoo... Since then nearly everything I have has gradually moved over to their products (head unit, trainer, fans) and I really rate them all.