Yes a 30w Co2 laser would be about right for 3mm ply, you could get away driving with that set up if bearings etc smooth.
Shopping for lasers probably means a cheap chinese laser and power supply etc, costwise building it yourself would be more expensive than buying a k40 40w c02 3020 system from ebay slightly over £320.
Buying tubes generally means the chinese on Aliexpress want courier to prove arrived broken before claim accepted, couriers are not going to wait for you to open PLY box then cardboard and remove packaging so its a gamble on it actually arriving in one piece. Cheap chinese lasers vary in quality and do not last last very long if run at anything over 60% power.

I had a friend who purchased the cheap ebay K40 model and the tube lasted only 2 hours, but with a claim and persistence quoting UK trading full refund obtained for him and they were responsible for collection of the faulty item within 7 days which they failed to do and have still made no attempt to do so, now his property. Only buying through Ebay and paying with paypal saved him getting ripped off claiming via credit card companies etc possible but a real pain with eBay was simple peobably many such claims they were on the ball anyway.

If you needed larger area than 3020 I would still go that route, play with it on arrival and make sure all ok with unit and then use parts or modify to get what you needed from your build, ensure cooling up to scratch running hot radically shiortens life of tubes but a 2ltr bottle of water frozen dropped in a bucket would keep you going for a few hours.