700-850Kg sounds much (without weights), my Mini is 700Kg and thats with the cast iron A-series engine and the engine bay stuffed with Turbo etc..
(3500£ for a subframe, revers gearbox and a exhaust manifold is alot of money...)
I think it would qualify as a "ombyggt fordon", but im not sure, you better ask SFRO.
(My Mini is registerd with other engine, brakes etc.. and about 100hp (back then the limit was 10kw/100Kg))