I have been investing in gear for a very long time. I have equipped more than one professional studio with all kids of cameras and glass and every kind of lighting system plus complete black and white and color processing and printing facilities back in the film days. I then, later on, had to re-tool for digital photography. When you are running a business, expenditures and investments can be very critical in that all purchases and upgrades need to be justifiable financially. Even if you are not in the photography business, I can rrealte a common sense approach for all photographers.
I'm sure y'all have heard it said, the old adage, that great images can be made with very simple non-sophisticated equipment in the hands of a talented and resourceful photographer. My first boss and mentor used to say that he could make a good picture with a pinhole camera and a candle! He used to exaggerate to make his points but for the most part the old adage is true. Oftentimes, it is surprising to discover some of the most iconic images of all time, made by world class "superstar" photographers were made with very well made and reliable but rudimentary equipment.
The first step in making upgrade purchasing decisions is some honest, hard, sometimes painful introspection of your own talents, know how, photographic savvy or lack thereof. If you have decent equipment and are still bogged down with unsatisfactory work, your money would be better spent on literature, classes, workshops and other educational endeavors to troubleshoot your issues and bring yourself up to a better level of performance with your existing inventory of equipment.
If, however, you really feel, deep down, that you know what you are doing and find that your present equipment is limiting you in what you want to create or it does not enable you to perform the specialized work you aspire to, then it's time to consider additional or better gear. Obviously, if you are a photojournalist or a sports photographer you may require longer, faster lenses or bodies capable of high shutter speeds, higher and noiseless ISO settings etc. As a architectural photographer, you may need wide angle and perspective control optics. You may need lenses or accessories more appropriate to the photography that you are doing like more or or more powerful lighting gear. You may need a macro lens, a specials portrait or soft-focus lens and the list goes on.
Sometimes renting infrequently needed gear or improvising with what you have on hand will give you more insight into what you are doing and you will know when it's time to make purchases and acquire the more specialized accessories that you need once the work is ongoing and starting to come in.
The trick here is not to buy equipment based on, shall we say. "pipe dreams" but for work that you can realistically get involved in, sink your teeth into and are actually going to do. You don't need a 1200mm lens to make portraits, unless you want headshots of ferocious animals. Don't try to kill a flea with an elephant gun- kinda thing! You would be surprised what some folks buy and never really use.
If you are in the photography business, time is money, so you may wish to invest in equipment that works easier, faster and more efficiently or will increase or expedite production. In theses cases, you must carefully determine if there will be a return on you investment in a timely period. A quality improvement can also pay off in profitable returns in cases where you can make larger and sharper prints or expand your offerings an thereby increase sales.
OK, If you are independently wealthy and can afford frequent GAS attacks, want to become an equipment collector or open your own camera store, I can relate to that, otherwise let common sense prevail.
In my own case, things are more simple. I just run all my intended purchases past my wife- she runs the office, does the accounting , writes the invoices and pays the bills. She's the president and CEO/COO. I'm the chairman of the board- I was kicked upstairs long ago- they just let me take pictures!