A few comments:
1. The tube should not be mounted vertically. Over time, tiny dust and debris from inside the tube can collect on the output window in the tube. This debris will be burnt by the beam and will obscure the beam. It is possible to run vertically but life will suffer.
100% wrong. There is no any tiny dust nor debris inside the tube. No matter what kind of laser it is. RF or DC powered.
I know many laser machines with vertical set lasers working well long time.
2. Get a good quality laser, I'd recommend Coherent or Synrad. They are pricey but you get what you pay for, they are easy to control and reliable. Also can be run with only DC power. If you want cheap, then go with the RECI or similar glass tube.
Synrad is better choice than Coherent but all RF lasers stronger than 50-60W also need water cooling and have quite heavy RF cables between RF Power Supply and Laser. Unfortunately RF cables are 2-3m long only so RF PSU need to be set on a gantry together with Laser.
3. I think 100W would be plenty for what you are doing. Synrad has a power calculator deal on their site that you can use to estimate power required to cut different thicknesses of material.
Good choice. But check price and weight.
4. Mount the tube on the gantry so it travels along with the carriage. As long as you aren't running crazy acceleration or feedrates, the tube will be just fine going along for the ride. Mounting the tube horizontal will require only a single mirror. Older Kern lasers did this and it works just fine:
http://www.laserresale.com/upload_pi...t1_large_6.jpg
It is better to mount Laser along the gantry. No need to move Laser when working X-axis. First and second mirrors can be set only once.
5. Don't use aluminum sheet directly underneath. Aluminum reflects the laser beam back toward the laser which can damage the tube. For cutting metal there are special filters which remove reflected energy to protect the tube. You want to have as little support as possible behind your material, it will adversely effect your cuts and can reflect the laser beam back at your part, scorching the back side. You want a porous bed so the laser can defocus and be harmlessly absorbed. Normally you will have a hard bottom made of steel or aluminum that is well below the cutting plane of the laser. Once the laser beam is defocused it is substantially less dangerous. There are lots of different "porous" beds. Aluminum honeycomb is one option.
100% wrong.
Aluminium DO NOT reflects the laser beam back toward the laser as long as it is not highly polished.
Laser beam is focused by the lens in a FL point. After the beam passes FL point it is distracted and gets harmless.
Of course you should use a kind of Honeycomb or so but only to avoid dirt on a bottom side of material.
6. Lasers make LOTS of smoke and fumes, especially when cutting nasty $h1t like vinyl. You will literally die if you are cutting vinyl without excellent fume extraction.
Cutting vinyls is completely forbidden because of an acid. Sulfuric acid will eat fume extractor, mirrors, lens, machine steel parts as well as operators eyes and lungs. This is chemistry.
7. Please take some time to read through the info here and elsewhere so you have an idea of what you are getting yourself into. Lasers are quite dangerous when handled incorrectly, be safe.