Yeah, armour is personal choice. Some guys love it some guys hate it. The rental shop probably has some very basic stuff too. Also, the risk level of commercial downhill tracks is entirely up to the rider. If you take it easy and take the easy lines then it is about the same risk level as an XC downhill. Obviously as you pick up the speed and start taking the more insane lines, the risk increases.

For protective gear, I reckon the priorities are:
1. Helmet, full face is nice for downhill but not necessary for non competition.
2. Eye protection, sunnies or goggles. Eyes are very important.
2. Gloves, try typing with even a little graze on the palms of your hands.
3. Knee guards, more of an issue on tracks with big rocks, or when running flat pedals with long, sharp pins.
4. Elbow guards, nice for tight sections through trees.
5. Back protector, a camel back full of water can do this job to a limited degree.
6. Upper body armour, so nice when take a spill, land on your back and don't even get a little bruise.
7. Shorts with padded hips and stuff, never tried them myself.
8. Full body armour, Bloody hot, very expensive and a pain for going to the toilet.

I personally like armour for downhill, but don't always bother with it. I just give trees a slightly wider berth when I'm not wearing armour.

Oh, and I'm not a bit fan of spending a lot of time in the air either, like to play it safe in that area.