Million dollar question... If you found something thats in very good condition, why would you be spending more? If something did break, of course it'll be more exp than a single turbo car. This applies to any twins. I dont have any issue with my 33gtr at all since buying it 1 year ago and running 250awkw now. You wouldnt be pushing 300awkw with stock gtr turbos at all. You'll be looking at 250awkw.
As BNCR33 said, changing turbos is an expensive process. GT-SS will be around 3.5k, dumps will be 500 and labour will be easily 1k. Thats 5k all up excluding your ecu, boost controller etc...
Maintenance wise it'll be the same as any other AWD cars if its mechanically A1.