It's obvious to say that greenhouse plants need water. But how they get it - and how much they need - will have a big influence on your level of effort in maintaining them.
Cacti, just as one obvious example, need barely any added water at all to survive. They are extremely efficient at retaining the water they receive through the roots. Most bonsai, on the other hand, can almost never be overwatered provided they have the proper soil for good drainage.
So, the first consideration in planning your greenhouse water systems is to decide what kind of plants you intend to have, now and later. This will help you determine whether you can be satisfied by bringing in a watering can once a day or whether you'll want to go to the extreme of having an automatic watering system.
Automatic watering systems can be similar to soaker hoses lying in or on the soil that slowly leech water into the earth. Or, they can be overhead sprinkler types that either spray or mist water onto the plants. Again, the type you want will depend on the plants you intend to have.
An intermediate solution is to simply have a spigot or long hose from the house that allows you to water by hand more easily than using a watering can. In this solution, an internal spigot is preferable so you can keep the door to the greenhouse fully closed while watering.
Piping water to the greenhouse is no more difficult than running standard lawn-sprinkler type PVC pipe a few dozen yards. Unless you have a very large greenhouse, standard pipe will supply plenty of water for your needs. The pipes themselves, and the couplers and controls needed, are inexpensive. The major expense (or difficulty, if you do it yourself) is digging the trench.
Keep in mind when you plan any kind of overhead system that mists or sprays water that some of that moisture is bound to go to places other than the plants. Benches, floors and other greenhouse components should be able to withstand wet conditions without causing problems.
Excess moisture will inevitably end up on the walls and roof, no matter what kind of watering system you use, if your plants are anything other than cacti. Water evaporates from the soil surface and leaves, humid air enters when you open the door. You'll want to plan for some kind of ventilation. You'll also have to clean the walls and roof from time to time.
Ventilation can be a simple fan, plus a narrow gap around (at least part of) the roof. Cleaning mildew and other consequences of moisture in the air can be done using standard products, but it will be done less often if you plan your watering system not to accidentally spray anywhere other than the plants.
Allow 10-20% of your construction budget for your plumbing system. Even after the initial build-out, you'll have to do maintenance from time to time. Pipes and hoses crack, especially in extreme temperature change conditions and valves eventually go bad.