Yes, I got my B525 from "Eternal Comms" (very prompt delivery), they seem to be the only ones that include what I call the "bunny ear" external antennae. Having said that, the built-in internal antennae on that router are pretty good anyway.
The extra £20 for the 65a get you mainly extra LTE bands (frequencies), so the router would work in more locations (in Europe + overseas), plus to my mind is a bit more future-proofed - for example, it includes the 700MHz frequency LTE band, and I've heard that Three have invested in this too (although it could be for future 5G deployments, not sure).
Yes, Eternal Comms advertised the 65a variant as having "Bridge Mode" - but they say the same for the 23a too. The only real difference between the 2 variants of the B525 (that I am aware of) are the extra 4G LTE frequencies on the 65a.
I think that, at the moment, the 23a would be just as good - it's just that I personally felt the 65a was more future-proofed, in case some of the other LTE bands become available here in the UK. But, other than that, I'm sure either variant (e.g. from Eternal Comms) would be fine.
I use the DrayTek Vigor 2860 Vac (the 2862 series is a slightly newer variant of that series, I believe) to do the Load Balancing between ADSL and 4G (I use ethernet + powerline adapters to bring the 4G signal from 4G router upstairs [will try in attic too] downstairs, and connect into WAN2 ethernet port of the DrayTek). Once I get the unlimited Three SIM up & running, assuming it's all ok (good thing there's a 14-day "cooling off" period, just in case!), I'll probably tweak the DrayTek settings to favour the 4G connection for most traffic, leaving the ADSL for some traffic where I'm not bothered about speed, and as a failover / backup link too.
I do have an external antenna: the Poynting 4G-XPOL-A0001 (which is an outdoor omni-directional antenna - I don't have any direct line-of-sight to any cell masts) - but, so far, the router antennae have been good enough to get a decent signal (far better than I can get on any smartphone) - although, once the weather improves in spring (hopefully!), I'll experiment outdoors with the Poynting antenna to see if I can get the maximum possible reception.
Anyway - I'd definitely recommend the Huawei B525
as the best 4G router (in terms of features & value-for-money) that I personally know of at the current time, based on my own experience so far (I know there's a Huawei B618, but that's a lot more expensive, and I don't personally think it would be worth the premium, except in very particular locations maybe).
Good Luck anyway - let us know how you get on!