I reckon you could get a custom built tank - custom built for you, that is - for less than £850, easily.
However, I guess the cabinet would add a LOT to the cost if it's also custom made and in solid oak...and from the sounds of it, this is the major plus of the set-up for you.
Could you try finding a joiner/cabinet maker local to yourself to get an idea of a quote from them?
Filter, I'd go for an external one as you can get a far bigger volume of filter media into it without losing tank space, maybe Ehiem; heater, I'd got for any decent brand.