I've found that Drei and Yesss! have relatively poor coverage in the mountains, including Ötztal (Sölden) and Zillertal. I get great coverage in the mountains using A1 B.free. At 1.5GB for up to 1yr for €20 it's a bit more expensive but IMO worth it.
Coverage in the mountains really depends on how far off the beaten path you go. If you stay on or near ski runs and built up areas, then coverage, even with Drei, may be good enough. If you go hiking or ski touring far from "civilization" then the carrier matters a lot.
Incidentally most of the cable car stations in Sölden now offer free WiFi.
As for buying SIMs, there's an A1 store a few minutes from the main Innsbruck train station. The main advantage in buying a SIM package from a phone shop is that the staff will usually help you configure your phone and activate the account.
As Vlad said SIMs, are also available at post offices. There's one in Sölden in the center of the village.
If coverage into Italy is a concern, you might want to contact the hotel where you'll be staying in Sölden and ask them. Or contact the tourist office
soelden@oetztal.com
P.S. If you have an Android smartphone you may find these apps useful:
iÖtztal
iSölden
Smart Ride