universal_exports ... it all depends on what you want out of a gun and what final configuration that you want it to be in as to which gun is the best way for you to go. You have to break the gun down into three categories. 1) Rear end only conversion. 2) Rear end conversion with front grip conversion. 3) Full rear end and front end conversion.
If you want a relatively true AK-74 look then that Waffen Works gun that you linked too is a great gun for the price. Bulgy gun with correct parts for under $650. Waffen Works is known for good builds and good customer service. The gun has no holes to plug/weld shut like a Saiga conversion because it is not a Saiga receiver.
If you take the IZ340 that Atlantic posted originally in this thread, the front end conversion is completely done for you, with the exception that the gas block will not have a lug ... which may or may not matter to you. Honestly, the front end conversion is the most difficult part of the whole thing. To get it in an AK-74 format you would have to do the rear end modification, adding $150 or less to the gun. So you are looking in the ball park of $625 or so for the converted gun. Depending on the look you want, you might want to plug or weld holes as well. Really, the price that the IZ340 is listed for is a pretty accurate price considering the cost that you would have in parts for the front end conversion to the base gun.
If you take the the bare bones Saiga, you can get the gun and do a basic rear end conversion for probably $500 or less. This will still leave you with a non-converted front end. Convert the front end and you are back at the $625-$650 ball park on the gun.
My recent conversion I invested about $750 in the entire gun, but I went for details that are not necessary.
Like I said, it all depends on what you want as the finished product and what your budget allows. If you want an out of the box AK74, then that Waffen Works is a heck of a deal, IMO.