1994 sts northstar motor

There have been changes. 93/94 same. 95-97 same except that 95 is OBDI and 96/97 are OBDII.

is there any defects in these motors is one better than the other

They are pretty much the same. The pre 2000 are more prone to a head gasket failure.

Moezy, Some of the answers to your questions are running in your other engine thread, right next door. If you discover that your current engine is toast, either overhaul it or find another and do a head gasket job before you raise it back up into the car. BUT, stick with a 94 STS engine so you don't get into different wiring connectors and engine management computer problems. The SLS engine is different.

so if i buy a used motor i should do a head gasket job to the used motor is that expensive and do i really have to do that

If you buy a used motor that you cannot test then the safe thing would be to replace the HG's while it's out.

I'm sure every engine you are offered will be a 60K engine that ran perfectly before the car was wrecked...NOT! How many 60K cars are still out there?

Figure any used engine that you cannot verify is 150K.

is replacing head gasket expensive to replace when engine is out

Jakes stud kit will run you about $550 I think. Add gaskets and such. I think I heard the price quoted by someone at $700 - $800. If you don't do it, you may have to in the future. It's just good insurance.