ISO 100 is the lowest you can set, but why do you feel you can only use 1/50 shutter speed?
And why are you wanting to shoot with the lens wide open? Why not stop down a bit? Most lenses aren't at their best wide open. f2 and f2.8 are better, and by about f5.6 many lenses are at their best IQ.
In most usual situations where an ND would be useful, we aren't trying to both use a low shutter speed and a large aperture. For example, if shooting a portrait on a sunny beach or out in the snow, when there is a lot of light, you might want to use a larger aperture to blur down the background... but would be free to use fast shutter speeds. Alternatively, if you were seeking to blur moving water at the seashore or in a stream/waterfall with a slow shutter speed, normally we'd be using a smaller aperture to get a lot of depth of field. Or, if wanting to pan a moving subject and blur the background, a slow shutter speed would be needed, but the aperture can typically be set smaller.
I'm asking the above questions to try to better understand what you want to accomplish... why you feel you need to do both.
That said, I'd usually steer clear of "filter deals". They are often not very good filters that might make a mess of your images. With ND fitlers, in particular, you have to watch that the filter doesn't add a color cast to your images. There also can be flare problems using cheaper uncoated or single coated filters. And the quality of the glass in a cheap filter can be an issue... cheaper filters might make images soft, can even cause focusing issues.
If you were able to shoot at a higher shutter speed, then it might be possible to use widely available 2-stop or 3-stop ND filter, which can be found in good quality, multi-coated (such as B+W MRC or Pro 0.6 ND or 0.9 ND, or similar)
You can stack ND filters, too. I'd only do this with top quality ones, though. And it can be tridcky or impossible to do on a wide angle lens, where the rim of the filter might cause vignetting. But with a 50mm lens, especially one designed for use on a full frame camera that's being used on a crop sensor camera, this shouldn't be any problem. If you had a 2-stop and a 3-stop ND, use them both to get 5-stops of light reduction.
Of course, there are stronger ND filters available. B+W makes ND 1.8 (6-stops), ND 3.0 (10 stops) and even stronger.
It might be difficult to find much selection of ND that are multi-coated. If you can only get uncoated ones, be certain they are high optical quality and be very sure to use a lens hood to try to avoid any flare (which causes loss of contrast and desaturation of colors). In fact, it's always a good idea to use a lens hood, with or without any filter.
Finally, you might also find a polarizing filter useful. Besides controlling reflections, they can be handy to increase saturation of colors, deepen a blue sky and make clouds "pop", even improve portraits when people have shiny skin or control reflections if they wear eyeglasses. You need a circular polarizer, in particular, with any auto focus camera. A C-Pol, as they are often designated, also reduces light by 1 to 2 stops, depending upon it's setting.
Those variable ND filters are pretty cool... but darned expensive! Some prices I see, in the size you need... 52mm "Fader HD" (I suspect it's not multi-coated, but there's no info anywhere)... are more expensive than your lens! Singh-Ray are even more expensive, but don't make one anywhere close to the size you need.