If it's a carb'd 305, A turbo will be expensive and difficult. You need a special "blow-thru" carb for it. If it's fuel injected, Most new add-on turbo systems will run you about $2000-$2500. If you are going to spend money for a new engine, Why not just buy a GM crate engine?
The 305's in 1984 were 145hp for the carb'd engine and 165hp for the fuel injected engine. Buy a ZZ4 GM crate engine and it will give you 355hp with a carb. Or you can install a TPI fuel injection system and get about 330hp. The engine will drop in and mate with the trans (or you could upgrade the transmission as well). You'll have twice the horsepower and everything will just drop right in.
As long as the body and chassis are in good shape, It will make a decent car.