I think Zax is on the money.
50-60 watt would be my minimum wattage if I had to build a laser again. 30 watts is really just too limiting due to time constraints. Whether you are a hobbyist or a production worker you will get sick of waiting for the cut to finish if you go too small on the wattage. The speed will be sickeningly slow.
If you are setting yourself up to do work for the general public then I would save up and buy an RF laser tube even if it took me 2 years to do so. If you are only a hobbyist then go for the glass ones as the cost is 1/10 of the RF sealed tubes.
If it's your first build then a cheap glass one might be right up your alley.
eBay for just about everything and its mostly cheap.
Don't forget to serialize the build.
Rich.
I am not completely useless.......I can always serve as a BAD example.