I would get it.
I have several "pre Spotmatic" Pentaxes and just love they way they look and feel in the hand. To me they look kind of like a Leica 111 threadmount camera with the addition of a penta prism - similar size and shape. The black ones are harder to find so if in reasonable condition $50 is a good price. Test the camera first to ensure its slow speeds work properly. It is common in cameras of this age for them to need a service. Perhaps someone else canadvise you on the cost of that if it is needed. The other problem I have sometimes noticed is a tendancy for the mirror to stick in the "up" position. Again this is usually through lack of use and a service fixes it. At $50 I would not let the need for a service deter me - but you may not need one anyway. But you may be able to bargain the price down a tad.
The S1a is one of my favourites and I have at least 3 of them - they are cheap to buy but lovely cameras. From memory the shutter dial only goes up to 1/500th of a second. But in fact if you move the dial one position over you will find it actually has an unmarked 1/1000th of a second postion. This is because Pentax wanted to sell a cheaper version of the S1 for the mass market. The easiest way was to remove the higher shutter speed marking (leaving the guts intact however). (I am pretty sure this is is the S1a!)
As to the specific lens you mention, I cannot comment.
The normal lens for this camera was an auto Takumar 55mm f2.2 or f2. (A later f 1.8 Super Multi Coated version would do just as well. Or even a 50mm f1.4). The f2, f2.2 and f1.8 can be commonly had cheaply on eBay - $30 - $50 for the later version of the lens and perhaps a bit more for the earlier auto-takumar version. That earlier version has a small sliding "switch" on the side of the barrel which is used to "cock" the aperture into the open positon - which would then stop down to its working aperture when the shutter is fired.
If you buy the camera I would go online and try to get one or other of these Pentax lenses as they are simply superb in both build quality and optics. It would be worth the added investment. Besides the old auto Takumar is fun to use.
BTW you often see this series of camera advertised as H1, H2, H1a etc. This is because in the USA they were marketed under the Honeywell brand. So if you see references of this sort on the internet there is no reason to be confused.
I cannot recommend these cameras highly enough. They are fun, pretty, extremely well built, robust and cheap to buy. Go to this site for more comprehensive information.
http://www.taunusreiter.de/Cameras/Pentax_Main.html
http://www.taunusreiter.de/Cameras/Pentax_Takumar_e.html