Should I buy an engine bike kit?

I have always liked cars and bikes and stuff like that however I have moved around a lot and never had a place to fix things however I am knowledgeable about stuff like this and finally got money for an engine kit do you think its a good idea to buy a kit finally?

I have been on my friends engine bike and helped him fix it however he and others have said that they are unreliable but that parts are cheap should I buy one? ask for more information!