Congratulations on entering the mad world of Burrows (and 8 Freight) ownership
I have personally been using these for the last 12+ years so know a thing or two about them.... The company I work with were one of the first to use these full time for transport.
Yours is quite an early one (so Norwich built by his local welder!). You can tell by the basket structure and some of the little bits and pieces that are on this. The hydraulic stand fittings were faffy on the early ones but were simplified on later versions.
I'll vouch that the bike was returned to Mike at some time and those repairs were done by his welding man (in his words). The worst weak points are those sections on the basket structure.
Wheel bearings are 16002RS by the way for the rear but the fronts are different (I'll have to check the exact size). Despite the appearances at first glance quite a lot of bits are also available off the shelf.
The drum brakes are Sturmey Archer items and not Sachs. It's been a while since I checked but I think they are 70mm front and rear. There was a time that Mike fitted 90mm versions up front, but they were too powerful and kept tearing the spokes apart on the wheel! You'll need a single sided hub for the front but they are available (for racing wheelchairs and the like).
One that I bought was also full of water. It took two of us to flip the frame upside down and tip the water out of the seat tube
They normally are pretty water tight though. I used a JD Tandem cover for mine when stored outside, that did the job.
The front monoblade is fine. Mike did tend to tweak these quite a lot so some were drive sided, others were on the non drive! Check the crown bonding on the steerer as this can deteriorate with age.
I might be able to supply quite a few spares as I have loads - including the chain tension roller, mudguard bits etc.. I also have one of these that I need to sell on with a written off frame but mostly good parts (long story), and a fibreglass cargo box designed to fit these. Shoot me a PM if you need to at any time.