I'd probably look at something more of a pre-made DAC than a DIY Arduino one, as you'll find writing the software by far the most complex and time consuming part of such a project.
Luckily you do have a few options on the cheap-ish side, such as the RIYA DAC's, etherdream etc.
If your budget will stretch, I would highly recommend the FB3 and QuickShow combo for what you're doing. I believe the etherdream and LSX might come in a little cheaper, but QuickShow is really designed to be well, quick and easy, heck it can control itself if you want, whereas LSX is a bit more advanced.
The good thing abut DAC's is they do not depreciate in price very much at all, as at least 85% of their price is actually in the software, and it's a very niche market. So even if you bought a FB3 and decided a year later your lasers are crap/aren't your thing, you should still be able to get almost retail price for it again.
Building yourself is very satisfying, but indeed it chews through money very quickly to do it right. I'm in around $3.5K to my projector, and it's only 1.5W or so if the green behaves.