Welcome to HHC!
Yes, it'd be fine to buy him from his previous owner. Just keep in mind that he may take awhile to warm up to you, depending on how much handling he's had. You'll also want to get a bag of his current food from that owner, enough for a couple weeks, and a couple of bottles of the water he's drinking. Slowly changing these two things to new food/water will help avoid stomach upset (rather than an instant change).
You'll need his cage set-up, including heating, hide-away, wheel, food and water bowls, whatever you're using for bedding (liners are a favorite on here), and other toys. You'll probably also want whatever foods you'd like to switch him too, though you'll want to wait a few days before starting the food change.
Read through the stickies and threads on here. There's tons of information that will help you prepare for owning one of these little guys.