You really need screwgates / locking gate climbing rated Karabiners for both Figure of 8 and the Safety line. But you could tie the safety line directly into your harness it's just a bit of a faff having to untie it when your abseil is finished before you scamper off to Abseil again....
Snap link Krabs are rated ( strong enough) for an abseil but the strength of any karabiner drops off dramatically when the gate is open. All the screwgate does is provide a positive locking mechanisim to prevent the gate from comming undone it's the same with Twist gates or the Petzl AMD locking krabs. So for load critical applications ( belays, hauling, securing your portaledge, abseiling) locking gate Krabs are used.
Using a single snap link for attaching a Figure 8 to a harness is not considered good practice. At a push you could use two snap links back to back.
I wouldn't buy second hand kit as you have no Idea what it's been used for prior to you getting it.