I don't have enough experience with UPS or AVR devices to be able to advise you on what to choose. However, this article has a good explanation of how they work:
http://en.wikipedia.org/wiki/Uninterruptible_power_supplyThe typical consumer UPS would be the "Offline / Standby" type. According to Wikipedia, it "offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery".
Here is a diagram showing how it works:
http://upload.wikimedia.org/wikipedia/commons/7/77/Standby_UPS_Diagram.pngAs you can see, the inverter and UPS are not the same thing. However, it is true to say that the inverter is the principal functional component of a UPS. It converts DC battery voltage to AC mains voltage.
As for the "AC voltage stabilizer", it is indeed the same thing as an Automatic Voltage Regulator (AVR). This article talks a little about them:
http://en.wikipedia.org/wiki/Automatic_Voltage_Regulator#AC_voltage_stabilizersHowever, the following diagram (for a line interactive UPS) better illustrates the autotransformer action:
http://upload.wikimedia.org/wikipedia/en/d/d6/Line-Interactive_UPS_Diagram(2).PNGAIUI, an AVR would consist of the autotransformer and nothing else. The above diagram shows a UPS that incorporates its own AVR. When the supply fluctuates, the AVR automatically switches over to the appropriate tap on the autotransformer, thus ensuring that the output remains at 120VAC.
As for your question regarding an SMPS that incorporates an AVR feature, I can't say that I've heard of any. However, those PSUs that accept a universal voltage input should be able to adjust to voltages in the range of 100VAC to 240VAC. These often have an Automatic Power Factor Correction (APFC) circuit.
Otherwise, even without APFC, a typical SMPS can tolerate a wide input voltage range. This is because the control circuitry uses PWM (Pulse Width Modulation) to regulate the output voltage. For example, let's say that the PSU is normally delivering 150W from a 240VAC mains supply. In this case the switching transistor ("chopper") may be switching on and off with a 10% duty cycle. If the supply then drops to 150VAC ("brownout"), the duty cycle may increase to 25% in order to maintain the same output power. In fact my own cheap AT PSU works quite well at 150VAC. I expect that my supply could also tolerate an input as high as 280VAC. At this voltage, the two 200V DC filter capacitors would be operating at their maximum ratings. This time PWM would reduce the duty cycle of the chopper transistor.
In answer to your question as to whether the SMPS can fail in such a manner as to deliver an overvoltage to its load, yes that is possible, especially if the filter capacitors have bulged or leaked.
Looks like this is more common than I thought in Seagate....
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