Ahhhhhhh Dacor Extremes...huge, heavy, but they do breathe nice. It's a shame they can't be serviced. (I have 2 sets bought in a used package deal....I only wanted the tanks) Actually, I believe that the parts can be found, they are just not available as a kit anymore from Mares.
Keep your eye on Craigslist. I have bought several complete sets used that only had a few dives on them. I did buy our regs first, and bought them new, just because my family was brand new and did not want to trust rental regs. I went with Hog/edge from DRIS because of price and reviews. I figured that the one piece of equipment that keeps you alive was not worth chancing on rentals. A regulator failure requires immediate attention or death is way too near. As long as your still breathing, everything else can be dealt with.
Also, my LDS will clearance out their rental/ training gear at the end of each year for around half price. Top of the line stuff that I trust has been serviced and maintained as needed.
3.5 Wetsuits can be found new for less than $100 so I consider that basic equipment like a mask and fins. unless your planning on local diving and need something thicker, even then I've seen 7mm suits clearanced in the $100 range. Swimming in someone else's peed in suit seems gross....swimming in my own peed in suit seems fine. Lol, I didn't say it had to make sense... Anyway, I would go wetsuit and regs first.
BCD's are so varied with so many options, it's easy to buy what you think you wanted....only to find out with a few more dives...you really prefer something else. Many here will say just start with a back plate and wing as opposed to a jacket style BC. This is one area that renting several different types may save you some money before you purchase. Rent, talk, research, rent some more, borrow, steal, etc...then buy. I'm juggling the choice between backplate/wing and a Dive Rite Transpac now to upgrade my jacket style Scuba Pro Glide X.
Tanks would be last unless you found a used package deal. As long as you have access to rental tanks readily available, and will dive locally frequently, owning tanks may not be necessary. The cost of visual inspections, hydro testing, and fills may make renting a better choice. We own 10 tanks, all bought used, so the cost adds up. Again, dive several options and decide what works best. I like HP 100 or bigger steel tanks best, but a heavy aluminum 100cf tank is a close second. I'm an Airhog so I need a bigger tank to dive with wife and kids so I don't cut their dives short. More air is always better than less!
If you have your old tanks, they may still be fine. There were a few years in the late 80's to mid 90's that an alloy was used in aluminum tanks that has been condemned. A quick Google search will tell you what it was. As long as the birthdate is 2000 or newer, you should be safe. Even if your tanks are in the range, it was a very specific alloy used. As long as they pass hydro and vis....and you can get them filled, your fine there. I have heard of some dive shops refusing to fill older aluminum tanks just because of age (and possibly stupidity) but have never seen it firsthand.
Good luck,
Jay