If you get a DSA or Imbel receiver on the rifle AND the rifle has not been build by someone who "modified the receiver to make it work" (which should never be the case, but....) You will have a solid "base" to work from if you have to make repairs to the rifle to get it to function. The other receivers either are low in quality or resale value, thus more difficult to get your $$ back if you try to sell (to fund your M1a).
922 compliance parts are key as well. If the seller can not identify the required US parts, I would either steer clear or plan on bringing the parts count up the next day.
If you have questions about what parts need to be US or how to identify them, either spend some time at FALFILES searching or I would be happy to reply to a PM (normal legal disclaimers invoked of course).
ensure the barrel is in good shape, no pits in the inside, no knicks on the crown. Seems to be mounted straight and gas block aligned with the gas piston (tough to eye ball, but if it is well off, steer clear)
Throat errosion and head space gauges would be very handy as well.
Some quick thoughts:
you can get all the US parts you will need for @150
Barrels can be found for @150, @250 new DSA, sometimes you can get deals
It will cost @50-75 for a barrel change and headspace by a good smith
US Hammer/tigger/sear combo will run you @55-80, depending on brand
Complete new Spring set will cost @30, little less
Complete parts kit for rifle for @150-180
Most orignal, foreign parts are rather inexpensive, however, you can not just use them to replace parts, as you might drop short of your require US parts count.
The typical US parts used to make the rifle legal:
Hammer
Trigger
Sear
Pistol grip
Forearm grip
Butt stock
Gas Piston
Charging handle
Muzzle device
Upper receiver (Only if a DSA, Imbel is foreign, the others I would not bother with)
The mag body, floor plate and follower all count, but I only know of US made floor plates. But if you go this route, the rifle becomes a felony as soon as you put an all foreign mag in it, so beware.
Or order a DSA, if you can get an Stg 58 kit gun from DSA you will be very happy with it.
Or order a Springfield M1a Standard, and not worry about the above
Good luck