The Allchin is a lovely model, by the same token there is quite a lot of detailed work in it which is not to be under-estimated. If that’s what you want then the price that you quote doesn’t seem too bad as it includes the boiler kit. I’d be a bit wary if buying another’s part finished project though, check the quality and extent but you may well be satisfied of course.
I used the drawings, which are virtually error free, I also had the Hughes 2nd write-up, (in ME c1976 IIRC). which is very comprehensive and very helpful, later published as the book and just might be sufficient to manage without the drawings but I wouldn’t really recommend that.
Don’t forget that you also need the gears, spurs and bevels, at around £500 if purchased from Reeves
As for timescale, I made mine many years ago with a fairly basic workshop, I might have logged about 2K hours, that was before time-savers like ready-cut wheel spokes etc. Painting and lining takes a while.
HTH
Can’t resist trying a photo in the new forum.