"Danger Will Robinson, Danger!" Putting a brand new motor, transmission, and drive train in a rough 1983 Vega is still a 1983 Vega. I don't want to burst your bubble, but the receiver will still say "C93" on it. So even if you put another $500 in it, most buyers will still see it as a "C93". If you want an "investment", watch for an actual HK 93, buy it at a good price, never shoot it, and clean it regularly. It would be very hard to get your money out of a corrected C93. Unless you find one for $200-$300.
IMHO, to buy a C93 and have it fixed, is because you want a 5.56X45/.223 roller locked carbine to shoot a lot and you want it to run right. So if you buy a straight gun in the $500 range and replace the barrel with a RCM nitrided hammer forged barrel, paddle mag install and refinish, you might have around $900-$1,000 in the rifle. Even after the refinish and it looks better than brand new, how many guys would be willing to give you the amount you had in it? I don't think very many. I wouldn't. I'd want to give you $100-$200 less than that. If I was willing to give you what you paid for it, I'd just go buy one and have the work done myself. To me an "investment" is something that is worth more than I paid for it, not less. In 10 or 15 years from now you might be able to get a little more than you have in it, if you don't use it and keep it pretty. But if your putting 500-1,000 rds a year through it, it will have dings, scratches and wear. It will be a worn C93. So chances are you won't get what you paid for it. To put that "investment" against a $1,000 T-bill, I bet the T-bill will have more monetary value in ten years.
Again, my intent is not to "rain on your parade". To me, it is to be realistic. If you want a good 5.56X45/.223 roller locked carbine, buying a straight C93 that has not had the bolt ground and have a RCM barrel installed will get you a nice one. There is an element that has value. The enjoyment of having a good carbine that runs. I can't put a dollar value on that, but it does have value to me. Good luck with your C93 project.
Scott