2002 honda civic, 174k miles, but its been parked for 5 months. Is it even worth going to look at with a mechanic?

I plan to drive it for at least 3 years(work/school), is the mileage too high?

Not worth it.

Is the mileage too high? No. A Civic can run to 300,000 or so if taken car of.
Is it even worth going to look at with a mechanic? It depends on how much it is being sold for.

At $100 it's the bargain of a lifetime. At $8000 it's not worth it. But you forgot to mention the price. Yes, I would have it looked at. Five months is not an excessive amount of time for a car to sit. If it ran fine before then, it's probably fine now.

It all depends on how well the vehicle was maintained before sitting for 5 months
Plan on needing the battery charged or replaced and some fresh gasoline should be put in the tank.
Once you get it running have the oil and filter changed right away.