Re: hi all
Welcome aboard. You can hardly go wrong for $75.
If a bike mechanic was having trouble getting a crank off, then I can't help but wonder if the crank or bottom bracket is damaged. But even if you had to put the engine/drive train on a different bike, it's still all workable.
The only worthwhile advice I could offer right now would be for you to be prepared to tinker with your bike and get your hands dirty. Every single one of these bikes requires it.
But it's more fun than not and you can end up with an amazingly competent vehicle at very little cost.