If you have ten years of building engines experience and combine the service manuals notes with some updated materials of course it would be a wise investment to build your own engine, even a stock one.
However many people lack the tools and experience
so you gota ask yourself. Do I feel lucky
oiling orifices in modern engines are so small the slightest bit of grit glob clogs a passage, ruins the engine.
If you buy one thats been exposed to atmosphere. Lets say I have a running engine and take it apart all oily inside, and I set it on a table top and leave it there for years.
After a year or two the sheer quantity of debris buildup inside the engine due to time and atmospheric entropy, interaction with myriad content and natural tendency of forwards reactions such as oxidation when exposed to long term storage/exposure as in this example renders the engine block useless forever. After just a short period of time environmental ingredients attack and stick to and alter the surfaces of an engine's internally exposed materials because they are intended to be sealed up with very clean oil, sealed perfectly away from the atmosphere and a wide variety debris. You can't just clean an engine thats been improperly exposed to certain conditions so being very selective is the main point when choosing a block with intention of rebuild and high mileage. Likewise never use an engine which has already had a catastrophic failure unless absolutely necessary.
In short, if you find an engine and intend to rebuild it, make sure the engine has never been open to the air (for very long), and has as much of track record for oil changes (visual indicators may help determine maintenance) and when you go to rebuild the engine if you have enough experience you will learn to use a clean room, air filtration, and build it as quickly as possible, then seal it up very tight and keep the oil clean using high quality filtration and pcv systems.
Novices can get away with slight rebuilds by following the service manual exactly and making sure to start with an already good running engine. But thats a different story than trying to piece one together... IF you got a running one that needed a rebuild with clean oil and no abnormal wear, it would be ideal as a novice to simply swap the bearings/rings hone and go, with no machine work and close cleaning attention to detail. However where will the novice acquire a cheap, running, high mileage sr20det engine without any issues or abnormal wear.... That was not the question being asked and doesn't seem cost effective, merely easier and higher chance of success. Which actually might make it more cost effective in the long run since failure rate is empirical for the person asking, I guess it still comes back to experience because exp leads to mistakes and If you've never made certain mistakes with sr20 engines (or any engine) You are bound to make them for the first time on the first attempt....
Many have tried building the sr20det engine, there are many mistakes to make
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