If you are sanding
the bottom of a Bristol 47.7, you will want to invest in the sander that gives the least amount of vibration, and, unless you workout daily, go with 5" because of weight. The additional amount of time you would save with a 6" sander wouldn't equal the amount of downtime you will need for recovery. My whole working life has been with power tools. Big box store tools quite often are a different model, built for pricing, than what you buy from a specialty tool supplier. When they are an equal, there is very little price
savings. Same with abrasives, what the box store sells is price
point, and because it wears quickly, you will be back to by more. A few minutes after you start sanding the bottom, you will look towards the remaining work to be done, and will be overwhelmed at the size of the boat.
As far as tenting, yes it will contain the dust to an area where it can be cleaned up, but it doesn't help with your exposure. Even suited up, you will have a problem keeping your goggles and face shield clear so you can see, the dust will get every where, even with the suit sealed. Using a sanding SYSTEM, like Fein or Festool, will minimize all that, by capturing the dust before it will be flying around.
If you are just scuff sanding to apply addition coats, use a pole sander like drywall finishers use. The bottom should be pressure washed, so all you need to do is sand it clean. Could do the bottom in less than a day. Lay a tarp down, choose a windless day, and the sanding dust will fall straight down for easy clean up. If you are taking it down to barrier coat or fiberglass
, don't even think about it, just go to a real tool supply, look and try the different sanders and vacs, and make your decision. I still stand by Festool, expensive, but you get what you pay for, and and they have some of the best abrasives made, specific to uses, and there abrasives are competetive with other quality abrasives.