The Arduino would be a good starting point for an altimeter, but I have found that they key to making a good piece of rocketronics is making the device impossible to damage.
The code needs to be hardened against any possible glitches (noise on the serial lines, glitches in power, false readings, stupid mistakes), and the hardware needs to be indestructible. Any type of non positive connector (like an ic socket, jumper setting, non screw terminal power connectors) will fail at an inopportune time, so you need to either eliminate them or design them to be fault tolerant (such as only reading the value at startup). It has to be able to be dropped in the desert sand and not be affected, be able to withstand the vibration/acceleration of the boost phase, the hundreds of gees that can be experienced during a hard ejection, temporary power loss, batteries being connected backwards, igniters being shorted, ESD to any exposed terminal, battery brownouts, switches being dirty, etc, etc, etc.
Even for personal use, all of these things apply. Just because you don't think you will have a hard ejection, doesn't mean it won't happen. I have had my trackers fail due to electrical noise, high gee rated connectors coming loose, mechanical stress snapping larger components off the board, battery brownouts, and a few times when they just gave up and quit in the middle of a flight.
<shameless plug>
But if you are true in your quest, you might check out my website
https://www.krazerlasers.com/gps/