When it comes to scuba equipment the saying, “buy once, cry once” holds very true.
I have 15-year-old regs (Mares Abyss) that still stand up to the latest and greatest, a backplate and wing that will never become obsolete, and a slew of old Oceanic and Aeris computers that time can’t seem to kill. The key is maintenance.
Whatever you buy, if you buy the right stuff, you may never need to replace it unless you just feel like it. Just take your time and do the research. Then be patient until the right deal comes around.
Also, used gear can be excellent, almost every piece of gear I own was bought pre-owned. But make sure whoever you buy from took good care of it.
With the exception of dive computers, in the last 20-25 years, dive gear has seen only very minimal improvement / change. There has not been any truly game-changing equipment developments in the sport. That should reassure anyone on the fence as to whether to invest in their gear.
My personal experience is that I enjoy diving more when I use my own gear as opposed to renting. Though ymmv.