Well according to your given options u can do either of them BUT consider this,
1 - if u put a second hand engine then u wont know wat's inside of it and how long it's gonna work before it gets siezed too.So If u wanna go for this option then take ur machanic with u and inspect the engine inside-out.
i - It's not time consuming as compared to engine re-build.
ii - No running is required.
iii - Performance will depend upon how gud the new engine is.
2 - IF u build it,then u'll come to know about the working and 'kastoriyan' of building an engine,which is going to help u in future incase of anything happens to ur re-built engine.
i - Re-building an engine is time consuming and after that the 'running' of engine.
ii - The performance of engine afterwords, solely depends upon ur machinc workmanship
iii - re-buliding will also give u an oppertunity for a little bit customization (IF u want) like Bore size,stroke,compression ratio etc
SO at the end it all depends upon u,wat option u avail.