To me, buying a R34 skyline now is the pure love and passion for the car.
If you want to buy it, you better be a big skyline fan. Cos maintaining it is confirm more expensive as no matters what, this car is already almost 10 years. Wear and tear is part and parcel of owning this car. If FC is even an consideration, don't even bother looking at this car. Cos this car is really a good drinker. At stock form for an ER34, best FC is 10km/l on very very very light foot. But if you wanna save on FC and go that light foot, you will be better off with a 1.6L. The 1.6L will probably be faster and yet better FC than the super light footed R34.
To buy a modded or stock R34, its all personal preference. Buy a modded one and you save the time and money to mod it. Buy a stock one and you enjoy the process of modding it to what you want. I will prefer a stock unit.
Maintenance wise, average out I will say about RM800 to RM1200 a month. For servicing, tyres and replacing wear and tear parts. Especially on the wear and tears on this Godzila. Tons of parts can just wear out without warning. And if you wanna mod it further, it all depend on how much is your budget. Modding this car is almost limitless. After owning this car for 5 years, I am still learning more about this car and how moddable it is. Every time you google it, you will find new things and new knowledge.
Above is just my 0.02 cent thought.