Buying used is a good decision for a new diver. Find a dive shop you can trust (that you may have done some dives with) and make it look like you’re interested in buying all of your stuff in one spot if you can get the right prices. Look for shops that have been under the same ownership for at least 15 or 20 years—in my experience they are a little less concerned about squeezing out every possible penny than they are about making a good impression and bringing back customers.
For a regulator, look at their line of rental gear. Sure it gets used plenty, but it gets serviced just as well. A good shop keeps their rental regs in good running order, and many will replace them on a yearly basis, selling them at the end. If you can catch them 6-12 months after they’ve been on the rental line, you get lucky making an offer. I got an Aqualung Calypso octopus with a backup reg, gauges, and a bcd hose for 300. I didn’t have to get it serviced for several months.
I purchased an old Zeagle horseshoe-style bladder BCD with noticeable wear for $100, and got it serviced for $40. Works like a dream.
As for a computer, I don’t know. I bought a new one though. Zoop novo