You have to buy a digitizer + screen assembly. The screen and digitizer are not separate. If you buy just the digitizer, it's a lot of hassle (read: about on par in difficulty with de-soldering a CPU) getting the old screen off, and it wouldn't be perfect when you put it back on.
But if you already got a new assembly, it should take about 30 minutes to 1 hour. Once you get the hang of it, you can do it within 10 minutes. It's not that hard.
I think your only concern should be that some third-party screens on the market are sub-par, and not all of them have the oleophobic coating or the scratch-resistant coating of an original screen.