Should I buy a mechanical chopper

Look for a strong motor and thick blades. If the blades are thin, they're likely to warp over time. The motor needs to be able to spin those blades through the liquid.

Look for a thick glass container. The plastic ones are unlikely to deal with the heat of things like soups and sauces being dumped in.

Check the gasket between the glass container and the piece that attaches the blades. A thin, flimsy gasket is likely to start leaking before a thicker, softer gasket.

Look at the number of teeth on the connection from the motor to the blade. The more teeth, the greater the surface area being used to distribute the power to the blade mechanism and is less likely to wear out.

Overall, as with many things, one of the quickest ways to tell a good one is to weigh them both. The heavier item is better. No one pays for shipping extra weight unless it's there for a good reason.

(note that the "heavier" rule doesn't apply to electronics like cellphones/computers, etc. where the cost of making it small is actually higher)

This is the cheapest chamber vacuum sealer that I've come across: VacMaster VP112. At $669, it's half the price of most other chamber sealers. Chamber models are far more "professional" than the stuff marketed to the home, and my understanding is that they are far more reliable.

The difference between a chamber model and a home model is that you place your food in a bag in a chamber. The entire chamber is used to create a vacuum, meaning that liquid won't leave the bag. A home sealer will also not go below atmospheric pressure while a chamber unit can get a much higher vacuum allowing for vacuum marination, compression or fruits, and better flavor penetration during sous vide, which may or may not hold any interest for you.

In terms of your specific questions:

1. My understanding is that these consistently make a good seal.
2. I can't speak to longevity, but these are the kind of units that restaurants use for sous vide prep, so I assume they take a beating in those environments.
3. Because it's in a vacuum chamber, liquid is no problem, and you can actually vacuum seal just liquid.
4. Major Fail. These things are big and heavy. The one I mentioned is 24" x 16" x 9" and weighs 53 lbs.
5. Shouldn't fail to seal. Not sure about waste if it does.
6. Cannot be used with a container, but why would you need to rather than using a bag?
7. Minor Fail. The model I listed is a little over your price range. Most other models of this type will be between $1,250 and $7,000.

The other consideration is chamber size. You obviously can't vacuum pack anything that's larger than the vacuum chamber. The one is listed has a chamber size of 12" x 11" x 5".