A 120mm fork will change the geometry of the bike quite a bit (the stock fork is a 63mm fork), and it will also significantly change the standover height. The front of the bike frame will be raised by 57mm (about 2-1/2"). This will be quite a change from stock. Suntour make decent forks when you go up in the line. XCR and Raidon are generally noted to be pretty good (both air forks), but I don't know about serviceability. Suntour get a pretty bad reputation for "cheap" forks due to the millions of trekking/comfort forks they make. And even those aren't "poor quality"...they're just designed for a certain use and a certain price point that isn't comparable to nice aftermarket forks. Suntour's aftermarket fork offerings are generally pretty good from what I understand.
Check out the sickbiker YouTube channel -- I think he recently did a video or two about the XCR air forks...possibly even servicing them.