There's a few things you're going to want to have for DH skating. I've listed these in rough order of importance (in my opinion). I've also added a few life-extending, budget, and repair tips so you can save your money for that set of precision trucks you need want.
Shoes
Make sure your shoes are flat-soled skate shoes. Traditional "skate shoe" type shoes are great. I personally prefer to have my shoes tight so that there's less slop between my foot, shoe, and board, so laces are a must for me. Additionally, leather or suede skate shoes will protect you much better if you go off the board and drag the toes/sides of your feet during a crash.
Budget tips:
Buy shoes from a thrift store or outlet mall to get lower prices
Repair tips:
Superglue / e6000 / shoe goo can be used to repair holes in the side or just glue down loose threads
Life extending tips:
Get a set of brakesoles and affix them to your shoe to improve grip on the board and life when footbraking.
Pants/Shorts
Pants and shorts are really a personal choice based on the heat level. However, you're going to be falling a ton on your lead asscheek (regular = left, goofy = right). I personally recommend getting a set of pants or shorts t that is cheap and durable. Think about thick materials like denim or canvas over light athletic ventilated fabrics.
If you choose to wear hip pads or crash shorts, buy pants a size up to fit over the shorts.
Budget tips:
Thrift some pants or get them from budget stores like Ross. Make sure to try them on and practice the positions you'll be skating in to make sure it stretches and fits in all the right ways.
Repair / Life Extending tips:
Adding a leather / durable fabric / duct tape patch on the lead asscheek will increase the lifespan. Additionally, putting a line of e6000 or shoe goo along the seams on that back pocket / asscheek will extend the life of the pants and prevent you from going cheeks out on the hill. Additionally, putting a line of shoe goo on the inner and outer seams of the seat of the pants can help those from tearing on a crash.
Knee Pads
Knee pads are generally required for learning, as you're pretty likely to bail and hit your feet, knees, and gloves. I prefer a hard-shell knee pad if I'm learning. The best kneepad is likely the TSG DHP downhill kneepad. These are specifically oriented towards downhill, and have a very solid securing mechanism. However, they are quite pricey for a beginner until you can afford these.
Life extending tips:
Duct tape is your friend. For basically every repair.
Other stuff
Sunglasses
Important for keeping the sun out of your eyes during long days on the hill. Also doubles as eyepro from dust and wind.
Sunscreen
Pretty self-explanatory - don't get sunburned.
