You haven’t given too much away in your thread about plant types, sizes, slant or slope of the garden, amount of lawn etc so in true singletrackworld style I will base my answer on assumptions and speculation!!
Plants in general are much hardier than most people think. Plants are on the whole prolific. Too much or too little of water, light, Oxygen, CO2, Nitrogen and others will impair growth, breed disease or kill them.
I think that your garden will love you more if you were to improve the soil rather than provide lots of watering. A fine earthy loam with plenty of humus / organic mater will hold lots of water anyway. Unless you have almost only annuals it would only need watering occasionally throughout the year and certainly not so much as to justify an irrigation system.
Hard to believe but I have found that watering too often promotes root growth towards the surface as the plants search towards where the water is coming from. Roots should grow down and out to search for water as that is where it is held in the ground naturally.
On the other hand if you are just after the okay to spend a bit of your money…….. go for it!!