I think it's better to get a new cat in the winter months. It's easier to keep it in if the weather is colder and the days are shorter. I've kept all our new cats in for about 2/3 months. They were only kittens though.
The longer it's in the better it'll bond with 'home'. By the time the weather is better it knows exactly who it's owners are and where it wants to come back to.
Usually the advice is to keep them in our or 3 or 4 weeks til they know this is home, food, fuss etc.
If very shy they might need longer to settle with you before heading out.
Winter is good, lots of reasons to spend time with you. I'd aim for a one year old or older cat as they don't need litter training and cats protection have loads of lovely felines. We got two around one or two years old and they are brilliant. In and out the cat flap all the time, but indoors and a lot more interested in cuddles in the winter the mercenary pair!