They are very reliable, i track mine and there is nothing that demands more than tracking. Keep in mind they get older and will require more maintenance but they are nothing compared to what other equivalent luxury car would cost. Even moded they are very reliable.
If you intend on modifying it, don't pay premium price for a X50 because you will change the turbos and intercoolers anyway which is what cost more on these car. If you don't work on the car yourself, dealer is VERY expensive, when i do a big job on mine, it cost at least 10 times less than what the dealer would have charged me. Wetune is good if you can't wrench yourself. Plenty of support on the internet also on 6speedonline and rennlist.
My friend is selling is 700hp 996 tt if you want one already moded and moded right for cheap.