The more new parts the better, in my opinion. There's nothing wrong with a new turbo on a used engine or truck any more than there's anything wrong with new tires. If the old one is worn out or failed, you replace it. How long a turbo lasts depends on a lot of factors. The most important being how good of a part it is to start with. The good OEM turbos like Garrett, AirResearch, Holset etc will last pretty much forever if they're properly "used" and maintained. There are OTR trucks out there with over a million miles on the factory turbos. The most common "failure" of a high-hour turbo is wear in the bearings that allows the impeller or turbine to start wearing against the housing and/or excessive shaft endplay. Both of those "failures" are usually due to cold starts, hot shutdowns and poor overall engine maintenance and operation.
When I needed a cheap turbo in a hurry I bought a 450,000-mile stocker from a mid-90s Dodge on ebay for $200. I then proceeded to beat on it pretty hard for at least 40,000 miles running up to 40 psi of boost and at least 10 psi pretty much continually at highway speeds. It's still in one piece. The engine is still in one piece but it's got a knock in it now. It was always very slow to build oil pressure at startup as long as I've had it, and I think the slow oil pressure and continuous boost did damage the engine. But not the turbo - it would still smack the boost gauge needle to 40 psi even after the knock started. Good oil and a little common sense in letting a turbo warm up and cool down properly is all it really takes for good turbo reliability.
That said, I wouldn't go to the extreme of the 5-minute "cool-down" timer a lot of guys run on their trucks. If you just spent several minutes at or near full load and boost before shutdown, then yes, a 5-minute cool-down is a good idea. For day-to day-use where the engine probably never even gets to FULL operating temp to start with, having your engine idle for 5 minutes before it shuts off is a waste of fuel. 20 to 30 seconds is MORE than enough for those situations.