The K-50 is the best buy for your bucks. I have the K30, previous model but inside it is the same camera, and it is a very good one, with all important features you need, wether it will be your first DSLR or you are already an advanced shooter.
K50 is at bargain price to make room for the newer K-S2, which offers a more recent sensor, with a few more pixels, a more recent and more powerful processor, a swivel back lcd and in-camera wifi.
K-S2 also has no lowpass antimoire AA filter, it being replaced by a commutable special vibration of the sensor to emulate the lowpass filter. This technology improves the sharpness, but it will only be noticeable when using high end very sharp lenses.
But if you dont need theses features, the K50 is in the same league as the K-S2. Its 16 MP Sony sensor is considered as one of the best Sony ever made, it scores almost the same as the K-S2 or the flagship K3 in DXO-marks.
The K3 is the flagship professional camera, thus better all metal built, rugged, silent, better ergonomy, much sharper sensor, fast burst and other pro features, but also heavier and more expensive; there are two models, almost identical, K3 and K3-II. In the K3-II, the built in flash has been replaced by a built in GPS with astrotracer function (that allows to track the stars for astrophotography), and an exclusive "pixel shift" feature has also ben introduced, that improves the IQ when shooting static scenes using a tripod.
IMO whatever camera you buy among K50, K-S2 or K3 should fulfill your expectations.
I have the DA 18-135, IMO it is the most convenient kit zoom, as you may face most shooting situations with this only one lens except extreme WA or extreme telephoto.
Though you can check my gear list is abundant, wether zooms or primes, the 18-135 is my favorite for travel or when I dont want to change lenses.
Though there are better lenses optimized for different situations, in which they will perform better, the DA 18-135 IQ is good enough to deliver very good pictures across its focal range.
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Tatouzou,
https://www.flickr.com/photos/70066783@N06/