The thing about Tokyo Marui is that they are an all around reliable beginner to experienced player gun. Their costs are not insanely overly high and the company uses high quality materials in their products and has very strict quality control. Which is why they make for great upgrade plat forms.
G&P is like TM in the way that they make great products with good materials and good QC, however G&P guns are steeper in price initially but you can find G&P models of guns that you wouldnt find in TM. For example the G&P UMSC SAMR.
If you started with a TM M16 you would require to replace the entire front end which would include barrel and RAS etc, the receiver, the front and rear sights and the internals. The only thing you would have left would be the stock. G&P gear boxes are great quality and their motors are fantastic. In the end its all about what you want to do and what you deem to be cost effective and how much you think your time is worth.
Me? I wouldnt have the time nor wish to waste the money to track down 25 different parts and pay to have them all shipped from separate retailers if I couldn't get them all in the same place. Purchasing a complete unit like that fit my criteria made by G&P or a similar company would be more effective.
Was there a specific question I could help you with in regards to the KJW M700?