You are dealing here with two distinctively different requests
1. To refurbish and upgrade the vehicle as such. There are quite a few businesses around specialising in various things, e.g. body , interior engine, gear boxs etc etc)and the best bet is to look in the internet or ask around in forums with specific questions (engine, body ??????)
2. To convert to the required off-road condition you need to approach a specialised firm of which there are quite a few around.
But your very first goal is to go for a properly refurbished, reconditioned and suitable standard vehicle (i.e. engine, body , interior, exterior, gearbox, etc etc etc???) and then go for the off-road additions/modifications.
I doubt very much that there are companies doing both simultaneously.
However if your vehicle is in excellent condition, you can of coursestart straight away with para 2. I would suggest tghhgat you look in the internet or alternatively buy a 4×4 magazine which contain many adverts of specialised firms.
Enjoy the upgrade and the travels afterwards