I agree with the others on the 'no perfect' life scenario but in order to be constructive there are some factors that i feel will usually make things easier.
Wanting children and being in a loving relationship were you both feel this way is obviously a great starting point and perhaps the most important thing not that there are any guarantees.
Generally speaking having a home and a certain degree of financial security will also be protective factors. Perhaps most important is to have good family/social support - this can prove invaluable during trying times. We're blessed with great grandparents and aunts and uncles on both sides that live close and can help out or give us the odd break.
I have what i consider to be close to a so called perfect scenario - married (or partner obv), chose to start a family, have home and jobs and super family support on both sides close by but it's still a big adjustment and can be difficult at times.
All being well in a general sense what's most challenging about first becoming parents is the 'constance' required in being a parent - You'll often hear friends and family say 'we love them and it's great to have time with them, especially when you know you can hand them back'. :-)
I can only imagine how difficult it may be for parents or a single parent to have a child in less than supposed 'ideal circumstances' not that those kids won't turn out just as great or anything. Not to be flippant but the 'perfect time' will be whenever they arrive. It's kind of got to be.