well it depends on the tools you have tbh.
Or have access to, there are hacker spaces with mills and cnc machines that cost as low as 30$ a month. So if you are lacking that can help.
I think its always better to build vs buy when you can.
Depending on if you want 2 or 5 W makes a difference.
For a 2.6W to 3W
1 9mm 445nm diode set to do 1W
2 635nm G71 diodes set to do 1W
Then green to get a perfect white is around 600mw to 1W
Some people when they see 1W of green with the other two say it to much others say its perfect.
If he wanted a 2-3W i would build but a 5W might be better to buy, as every thing gets harder.
A thing to note is dont skimp on the laser power supply's, a good high quality DIN rail psu was the best thing i ever bought for my projector.
Have yet to kill a diode after switching to one of them.
The 1W 445nm build is very easy just a diode holder and correction optics holder no mirror mounts needed or cubes.
The 1W 635nm build a tiny bit harder but you need a mirror mount to stack the beam one on top of the other then send that beam into the correction optics.
Now to do more powerful versions its just build 2x of the above and PBS cube+ waveplate them along with what ever green you get.
using the cubes is not super hard but it adds another set of things to think about.
Below is a bit of a different take on the above, in that the red and blue share correction optics. for your first build i would have each set with their own.
But you can see each laser is colored and even the galvos and galvo amps and psus are in there, blue is the laser psus DIN rail size and silver was the psu that came with my galvos.
The only other weird thing was my diy green in there thats a holder for a 22mm green laser. you can buy a 400-500mw 22mm green for about 200$
But with the prices for a green laser down to a reasonable mount i think i heard someone say they bought a 700mw green for around 350$ or less