It will perhaps be much easier for you to buy a used FJ40 and then make changes to it. Its very hard work making / restoring an FJ from scratch and ends up being costlier most of the time. Buying a simple 2F and then changing it to a 1UZ later on at some stage is a good option. There are some good vehicles for sale that you can choose from.
There is one particular FJ that I would recommend on the basis of 'comfort' alone. I have played around with FJ suspensions a lot but this one actually surprised me just with adjustable Ranchos and reduced leaf springs. Fuel average depends to a fair extent on gearing of the diffs but 6-7 km/l is normally achievable with 3.7s and a light foot. Nevertheless, an FJ has its limitations and will always remain a stiff vehicle in comparison to even a CJ7. Its big fullsize TLC diffs are the main culprit on both comfort and economy, although smaller prado/hilux size swap is always possible.
Don't pay heed to the naysayers and you can take it from me that (i) the FJ40 is and will remain an iconic car across the world, and FJ owners/drivers are not really known to be dumb or to be just in it for the novelty factor (ii) there is no vehicle that compares to a UZ in a 40 not just for driving fast but on daily drives too. These are Lexus engines at the end of the day and its not surprising how naturally they transition between wild beasts and tamed refined creatures. Only ones who have experienced it can really speak on the topic, the rest have not much clue what they are talking about.....
Nevertheless, you need to make up your mind and decide that you actually want to do this.... and BTW starting off with 'demands' from an FJ40 is not a good idea