If you’re planning on building one up and looking to save some money, go with the Imbel gear logo metric receiver. If you have a few more dollars (read hundreds) go with a DSA receiver.
The downfall to either one of these options is that you will also need to get enough USA parts in the rifle to make it legal. If you start adding up all the money that you will spend to build the rifle, you will notice that for around $800, you can get a DSA STG-58a that is already built, and has a warranty from DSA. The like new parts kit, the receiver, the USA parts and tools needed to build up just one rifle will run you well into the $800 mark. The real savings isn't achieved until you start building up the rifles by the handful, or until you find someone that already has the tools that you can borrow.
As far as going low budget, you could go with a G-1 kit, on a cheap FAL receiver, but be prepared to have to change parts and tinkering to get the rifle up and running. Some of the cheap G-1 kits are just plain worn out, and a few of the cheaper receivers can be a bit out of spec. Granted that if you have the tools, and the talent to go this way, you can keep the cost under $400, but don't expect the rifle to be anything more than a plain shooter with group around the 4 MOA mark at best.
P.S. FAC has the DSA STG-58a rifles listed at $795 the last time I checked. Add shipping and a $20 FFL fee for the transfer and you are out the door at for around $850. All that would be needed is to find a local FFL to do the transfer and pay FAC for the rifle on your own credit card.