If you can swing $175, you can get a decent new nitrox computer.
Mares Puck Pro Ultra Lite Dive Computer
User-Friendly Dive Computer for All LevelsThe Mares Puck Pro Ultra Lite Dive Computer is designed for both entry-level divers and seasoned professionals seeking a reliable, straightforward dive...www.scuba.com
If not, keep looking at used. Anything with nitrox capability will work fine. More expensive computers have nice features, but you don't actually need anything beyond a basic nitrox computer unless and until you get into tech diving.
Do make sure it's rechargeable or you can change the battery yourself though. Some older models required you to take it to the dealer for an expensive "service" which just meant using a proprietary tool to open it so they could replace the o-ring and battery.
^ This...
As much as it pains me to say Mares (I had a bad experience with them years back over their dive computer support), this is a pretty decent computer to do recreational dives with. And, It won't break the bank... I started both my wife and daughter with a Mares Puck (original model), and it served them okay.
I'd still likely go Deep6 over Mares...... but that price gets you going....
You need to decide what features are "must haves", "wish for", and "I'll never need."... That will drive your budget.
I own a bunch of computers because over time, things became important to me to have as features. Some times I had to buy a lot more computer than I was needing to get features I wanted. Thing is, all of them remain functional, and some are nearly 30 years old...
Cochran Commander Air (replaced with an EMC-20H when it bricked)
Mares Puck (used by wife & daughter when they started)
Oceanic VEO 180Nx (picked up used in a deal for some gear)
Shearwater Petrel1 (I went down the tech path)
Shearwater Perdix AI (have 2) - wanted AI and compass
Deep6 Excursion (got it when they were first offering them at an introductory price I couldn't pass up)
Shearwater Peregrine (have 2) - Wife & Daughters (no AI when I got them) - picked up both used
The Shearwaters are my choice for readability, and I value that as my eyes age...
