I'd suggest a Mach-Zehnder interferometer configuration because it may be easier to align.
Before you go out and spend all your money on developing an ultra-stable source, why don't you try a simple, free running He-Ne laser. I am suggesting a He-Ne laser because the output mode is very clean and this is important in practical interferometry.
If frequency of the laser is a problem, you need to figure out over what timescale are your measurements being made? Typically, gas lasers are stabilized externally by locking their frequency to a cavity (the Pound-Drever-Hall technique is well known).
If you are using a diode laser, things get a bit more complicated. You will first need an external cavity configuration (check out Using diode lasers for atomic physics by Wiemann, Hollberg et al.,) along with precision current and temperature control. Typically free running diode laser linewidths are in the order of 40MHz and with grating feedback, you can narrow the linewidth to about 1MHz. But without thermal control, the wavelength can drift by several nm. If you really want kHz linewidth and long term stability you need lock your laser to an atomic line and use active stabilization.
The point I am trying to make is that, the more demands you place on the source the higher the complexity of the setup. So, I'd suggest trying with the simplest available stuff, hell even a laser pointer to see what you can do with it.