High powered blenders (e.g. KitchenAid, 1800watts, 3.5 HP peak)

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)

I don't think it's possible. You are talking about restaurant / bar quality blender (like Vitamix, Waring, or Blend-tec). Cheaper units will not have the horsepower or quality of construction to do things like smooth blends of ice in a drink, generate enough friction to heat a soup from scratch, or grind flour from nuts and grains.

Even if your blender will handle some of these things initially (like blend ice, I'm skeptical any cheap mixer will provide enough heat for a soup), I think your blade / motor will quickly take enough abuse that it won't be able to continue to perform.

Update: Since initially posting this answer, I've actually bought a Vita-Mix (I'd been lusting for quite some time). I now feel more strongly that my answer is right. I've used it three times in the first 12 hours of ownership:

• Nut Butter - I made a pistachio nut butter that required nothing more than 2 cups of pistachios and a tablespoon of oil. It came together really quickly and has a lovely velvety texture.
• Pureed halved peaches - these ended up complete liquid with no texture / fiber left from the fruit. Exactly like Looza Nectar, but fresh.
• A smoothie. The smoothie had zero ice chunks in it and was professional quality.

I have never seen a cheaper blender capable of the quality of any of the first three things I've tried. I'm afraid you're out of luck.