You don't need to go further than a King Arms M4 or WE M4 AEG. Both are SAF out of the box and both go for $250ish. Not a bad price at all for their performance. Easy guns to upgrade (the KA being a bit easier) so you can build onto them.
Guardians of Asgaard
"The judge meeting is arranged for terrorist." - G&P Laser Product
Primary - Z-Custom VFC ZTW
LifeLine - TM Glock G18C