The transportation options in and around Guardea are: bus, train or rental car.

Buses
The use of busses will require patience as they stop many places and therefore often take a long time.
For schedules and prices you may use f.ex. Rome2rio.

Trains
The closest station is: Alviano
It is about 10 minutes by car from Guardea.
NOTE: You will NOT be able to find a taxi, so you have to arrange transportation in advance.

There are frequent train service to Rome (f.ex. Termini) and the ride takes a little less than 1 hour. 
Last train from Termini, Rome back to Alviano station is around midnight.

For ticket booking/payment you may use: TrenItalia (App or website).

It is possible to catch high speed trains (f.ex. Italo or Frecciarossa) from the town of Orvieto (some 25 km from Guardea).

Rental cars
No doubt that the rental car offers the highest degree of freedom, but it is also the most expensive.
Best advise is to use Rentalcars.com as reservations made through this company can normally be cancelled without any charges.
The only other thing to say about rental cars are:
- Check carefully for pre-existing damages. Consider making a video of all surfaces, rims etc and have the rental car company add any damages you may find to the rental agreement.
- It is highly recommended to use a genuine credit card as car rental using a debit card will be very expensive or, worst case, impossible.
- Consider insurance, your credit card may provide some kind of coverage.