I would suggest the DiveRite NiTek Trio (aka: Apeks Quantum, Cressi Giotto C3, Tusa IQ750, Zeagle N2ition, all made by Seiko). This model is a 3-gas nitrox computer that can be had for $350 new or as little as $150 used. This model will work fine as an air computer, then support you as you graw into Nitrox and even as far as deco on richer Nitrox blends, all at a pretty low price point. It also has a gauge mode which makes it a simple bottom timer (no alarms, calculations, etc.) which becomes useful if you move into mixed gas diving later or buy another computer and make this a backup at some point.
If you don’t want to go that route, I can also recommend the Cressi Leonardo. It has more or less the same display as the others, but supports just one gas (Air). It, too, has a gauge mode, so when (if) you move to Nitrox later you can buy a Giotto C3 (or other brand above) and put the Leonardo into gauge mode as your backup. This will allow you to retain the familiar screen layout (and have the same layout on primary and backup) while moving up to more advanced diving.