About the cost of a donor car usually as you would pick up the wiring harness etc. you can wring some reasonable power and torque out of them if you spend wisely but as other point out LS upgrades are a deadset no brainer these days. If you're going to run the injection you'll need a good wiring harness and peeps seem to charge whatever they want for them nowadays (I do have one somewhere but not sure of its condition or if it is an auto or manual trans one). If you don't have the wiring harness then it costs similar dollars for a Harrop dual plane inlet manifold and a small 4BBL carby.

Should it need one then freshening up the bottom end and if it is a 'blank' block (on the outside it wont say 5.0 in the usual spot it will just be blank which is an indicator of a relieved block able to easily take a Harrop stroker crank without huge mods/grouting) then a stroker crank is a good cost effective way of upping the power (and more importantly the torque output) with the standard cast iron heads.

You'll need a Delco 808 computer and memcal (the brains of the computer) or some other type of aftermarket ECM to run it.