I am about to buy my second set of scuba gear so am thinking about just this issue.
First time around - many years ago bought a farmer john used that was a very good fit and cut bits of it up and reglued it to be a good fit. Then went to my local dive shop and bought the jacket and hood custom fitted. It doen't hurt that the LDS manufactured Whites suits in the back of the shop at the time.
Can't imagine diving cold water in an off the rack wetsuit. Not with my pipestem wrists anyway (took them three tries to get the wrists right - the cutters couldn't believe the numbers the fitter gave them!) Then bought everything else used over the next couple of years. BC first then reg then tank. Was a student at that point and $ was a huge issue.
I still think this is the best order - if you plan to dive cold water, thermal protection is number 1 and with a wetsuit fit is everything. You want a suit that lets in the absolute minimum of water. None is perfect, but impossible particularly as you go deeper and the wetsuit compresses. Rentals are never going fit for obvious reasons.
If you are not comfortable ie cold you are not having fun and frankly isn't that the point?
The order of the rest is not so critical, I kind of like my own reg more than I care about the BC, but if I have my own BC I am a more comfortable diver so there is is a trade off. Also the BC you choose can have some influence the choice of reg configuration. Length of hoses etc.
Now that I can actually afford to buy what I want, I am still buying gear in that order. First the drysuit, then figure out what BC I want to use with my drysuit and then buy the reg etc. to match the BC I decide on. Computer, lights and all sorts of other goodies come at that point as well. At the moment I doubt that I will buy another tank. You can't use it on holiday dives, you can't use it on multiple dives unless there is air right there and if there is air right there I'm going to rent anyway. But I drive a small car and live in a small space so storage is a major issue. (Wondering where I am going to store all this gear - maybe hang the drysiut on the skiis and the BC over the golf clubs???)