I'm confident you'll make the best decision once you've got a few miles on the car. You'll know pretty quickly if the engine needs work or if it's up to the task.
How's the compression?
How clean does the oil stay?
Does it overheat or run overly hot?
Odds are if any of the above exist or check bad, you'll really want to refresh the engine before doing any modification to it.
Or just say hell with it and buy a new reman off ebay, if they're still on there.
Just be aware that turboing a Focus is not cheap like a Honda. At minimum you'll be spending around $2,000.