I have rebuilt an O3-A3 and had so much fun, that I have decided I don't care a lot about a few dollars one way or another.
My approach will almost certainly be more expensive than others, but I look at it as entertainment, not investment.
I am a few steps ahead of you on this project!
This is what I have done so far. Got set up with CMP and ordered a SG SA, a Greek H&R RG, and a stripped SA receiver. The receiver got here today -- it has a January 1944 serial number. The other two rifles are on back order -- I would expect the Greek to get here in 1 to 2 months and the SA to take about 6 (if they don't run out before then!).
My plan, such as it is, is to 1) Leave the SG SA pretty much as is when it gets here, other than clean up, replacing any questionable parts, etc. 2) Send the Rack Grade Greek to a reputable gunsmith for a complete re-park/re-build/re-stock to make it look and function like new, and 3) over time, purchase all the parts and tools needed to build up another one on the receiver, doing it myself, except for the bbl. installation - think I will let my gunsmith handle that.
This ought to keep me busy for a while!!!