The only real issue with buying gear immediately is your dive goals may change, your expectations may change. If you're okay with maybe "throwing money away" then buy the basics. A BCD, regs, computer (maybe), mask, fins, snorkel, and exposure protection. If you're worried about "wasting" money on gear you'll replace, then don't buy your own gear just yet.
I bought a used set of gear before I finished OW class. I still have all of it except the BCD, because I found a backplate worked better for me than the vest style BCDs. A lot of people end up replacing all of the gear they bought early when their diving style changes or someone convinces them the gear isn't good enough or they "outgrow" it or whatever.
Let me make clear ANY regs you buy will be sufficient for all but the most extreme (ice) diving so don't worry about buying the $1000 reg set when the $300 set is equivalent in the water. Don't buy $150 fins when $50 fins work just as well (unless you want to spend the $150 for some reason other than "performance").
Educate yourself about the marketing BS before you let any shop part you from your money. There are a lot of good shops out there but they are all businesses and they all have significant markups well beyond reasonable on most things you need to dive. A $100 mask is a rip-off in my opinion but some people believe they will fit better. I have an apparently easy fit face because I've never tried a mask that didn't fit well enough to dive with. As a result I buy used masks or $5-$10 masks or I buy the "snorkel kits" you find at Costco and they work great for me. $30 for mask fins and snorkel out the door. Hard to beat.
The reality is what you "need" will change depending upon where and how you dive. The basics are the regs, exposure suit (as appropriate for where you plan to dive), mask, fins, snorkel (for some people this isn't a "need" but it's required for the course, typically), BCD (also not a real requirement but it makes diving "easier" and some operators do require them), bottom timer of some sort, and of course tanks. Every dive I've been on with a shop provided tanks so those aren't a requirement to purchase, obviously.
Beyond those things, it's all preference and marketing.
A lot of folks also carry an SMB (surface marker buoy) for safety and some sort of cutting tool or two like trauma sheers or a small knife. Again, not requirements but a cheap way to add another layer of safety to your dive kit.