I'm really looking for a fast and silent motor/servo.
You really have at least four variables working against each other here:
Fast, silent, able to move a lot of weight ===> $$$$$
...and that's just for the actuator - the cost of the power supply is also going to be relatively high as well.
That said - have you looked into Firgelli Automation products - specifically their linear actuator line?
For instance - here's an actuator that can move 100 lbs (approx 45 kg) fairly quickly:http://www.firgelliauto.com/product_info.php?cPath=114&products_id=246
$169.00+ USD isn't cheap - and that is only 1/3 of what you need for your spec; they sell ones that can move the weight you're asking, but they are more expensive - I also didn't see any that were fast. Typically, with electric-motor linear actuators, speed trades off with the amount of weight it can move. You'll also need to shop carefully to find one that has an integrated potentiometer for position feedback (plus the costs of either a servo interface, or building one yourself - for each actuator).
A different option (but still fairly expensive - unless you get lucky; I suggest checking ham-fests and the like) would be to use two electric power-chair gear-motors (such as are used for differential-steered chairs). Mounting a lever on the end of the shaft (that, or some other rotary-to-linear conversion mechanism) in place of the wheel will probably be tricky. The motors typically run on 24 VDC, and require extremely hefty h-bridge controllers to handle the load - but they are very quiet, fairly fast, and can move the weight you are specifying needed. Depending on where/how you find the motors, expect to pay anywhere from $100.00 to $200.00 for the pair of motors. Or - if you are really lucky, sometimes you can find them for much cheaper - search for a whole chair online @ craigslist; also shop at thrift stores and similar (I have found full chairs very cheaply before at Goodwill; once, it was half-off Saturday, and I could have bought a complete chair for $20.00! The only reason I didn't was because of how much the chair weighed). Note that if you go this route, you'll have to come up with your own feedback system to implement a servo feedback loop (PID or otherwise).
Another option (not really recommended, though - and likely to be very expensive for all of the parts) - would be to use electro-hydraulic (and stick the pumps into a sound-deadening box, another room, outside, etc). The hydraulics will run you quite a bit, and hydraulic fluid leaks can and will be a problem indoors; again, you'll need extra parts for the servo loop...