Totally agree with Mr Ed. I've replaced all electrics, nut and bridge on my '65 Epi SG and had it professionally set up. Sounds and plays great now!
However, it adds significantly to the cost. I've spent the same as the cost of the guitar modifying it. However, it's still a cheaper option than upgrading a more expensive guitar and you can do it in stages.
Bear in mind that Epi build quality can vary considerably, so play several and make sure you get a "good one". I've not played an Epi SG as good as mine yet, and I've tried loads for comparison purposes.