The Honda eu2000i inverter generator seems to be the standard on which others small generators are judged but they aren't cheap. It's also very quiet to begin with.
Buddy I know from church has an older 3500 watt Champion that we converted to burn propane. He originally bought it for emergency use but it also runs his table saw and other power tools in his detached garage which has no electricity running to it. The Champion 3500s have a fairly good sized muffler with a flange on the outlet but are still somewhat noisy being an open-framed "conventional unit"
I'd look for something in the 2000-3000 watt range for your application.
Inverter-type generators like the Honda eu2000i work by generating a high voltage, 3-phase AC current at a variable frequency which is then converted to high voltage DC then back into a stable 60hz (or 50hz depending on your country) AC by the inverter electronics. Since the output frequency is independent of the motor and generator head speed, they can operate the engine at lower speeds which helps reduce noise and conserve fuel. The design and the inverter electronics adds to the cost (and the complexity).
Conventional generators, on the other hand need to run at a certain-fixed speed (typically 3600rpms) to maintain a stable voltage and 60 or 50 hz AC output frequency which is why they are typically loud. Being less complex they are also less expensive and more common.
Not too keen on running a generator in an attached garage even with the exhaust routed (possible leaks) but a small "doghouse" type enclosure outside would also help dampen the noise.
The Honda eu(x)i units are already quiet to begin with but in Japan, they do sell something call the "Silent Box" for the eu16i (Japanese version of the eu2000i) that the generator can be stored and operated in and it supposedly will make an already quiet generator even quieter.
http://www.honda.co.jp/generator/option/silentbox/