Electrician and Plumber are needed. Companies offer a couple kinds of online generators. Generac is a big name around here.
Ours is a Generac and run on propane. Throwing overspeeds, overcranks, general upkeep, tune ups, etc.. can be costly. Then there is the cost of propane. We own two 250gal tanks. Power losses are long here... I go through about 2 tanks for 10 days of running it. We conserve by running it manually when the power is expected to be out longer and we can't always get a propane delivery in time.
All in all the Generac we have was ~$7K and the tanks an additional 1500$ plus propane fill cost$. Service contracts are about $300-5$00 annually.
In my next home, I will have a less expensive portable generator and underground power lines.