Since everyone on here thinks that motor swaps are the only thing to do...I'll go against the trend. If ou're not looking to do anything major to your car (ie: forced induction) then I would just rebuild your motor. Essentially what needs to be done is ALL gaskets should be replaced, everything should be checked to see if there's any excessive wear (pistons, bearings, crankshaft, etc.), and compression should be checked. Also, check to see if the head is warped at all.
If you're doing it yourself, I hope you really know what you're doing and have more than just a Helms manual and a ratchet.
I personally wouldn't feel too safe doing it myself, so I would take it to a shop. Depending on how beat your motor is it could cost anywhere from $300-$500 (my best guess).
SO, if you want to go buy a swap and messwith wiring harnesses and shit like that, go for it. It doesn't seem like you're modding the car much at all so I don't think it'd be worth it. If you buy a used engine w/ low mileage you run the risk of finding out it was someone's beater car and they drove it into the ground. My advice is take it to a shop and ask for some kind of estimate...then go from there. Hope I helped and good luck!