First off, the 2.5 is just as bad from hawk onto even the final edition, doesn't matter if its wrx or sti, they all suffer from ringland and headgasket issues.
Subaru stopped doing a proper forged engine in 2001 (well before the 2.5 came in ironicly)
Modified and standard doesn't make any difference up to a certain power, over 350bhp and your on borrowed time on a stock 2.5.
Well looked after or not (although poor maintenance will always result in a less reliable car) they still fail.
I hear a figure of about failure rate of around 35%. which is pretty astronomical in car terms but in truth it's a educated guess. Really its pure luck if you get a good or bad one. I know of at least one car that had 3 engines within 10k miles from new 😮
That's the bad, the good is built right the 2.5 is an epic road engine, plenty of low down grunt. Well looked after and tuned right they are proper weapons on the road.
Proper forged rebuild will prob start around 7k ish for a proper decent basic forged build (assuming you don't do it yourself) with an upper limit really depending on how far you wanna go as far as power goes.