Speaking as a mother who considered having DS in with me (with the blessing of my office mate!):

You'll need a good day-sleeper. Which generally means a bad night-sleeper so you'll be knackered.

You'll need space and an easy-to-carry playmat, a baby won't be worn all day. Most playmats have noisy toys, so be prepared for a day of "hickory d1ckory dock". Babies like to play with your things so get ready for an attack on your computer!

Smelly nappies - can you change in the loo or is there no room? Smelly nappy in an office... ewww!

I'd say you could maybe do a couple of mornings or afternoons with a baby, not more than 10 hours a week in an office, assuming no colic, you're feeling OK, no reflux, no anxieties... even a "good" baby can't do a 40-hour week with no stimulation! Don't forget the hour-long feeds at first, you can get loads of typing done but not so much running to meetings. And if the baby sleeps and the phone rings, do you comfort the baby or answer the phone? I'd look at working from home, sharing care with your DH (maybe you could meet for lunch and he take the baby home, you work the afternoon without the baby) and even doing unsociable hours so you're not as disturbed or have people complaining that the baby is a distraction.

I wouldn't have DS with me. He needs me too much. Yes, I can get him playing cars and his "brrrm-brrrm"ing isn't too distracting, but he'll not do that all day. I need my computer for me - people don't like letters with baby-typing in! He's not only "good" but bloody fantastic, even so he couldn't do more than 2 hours without me tearing my hair out at the office - but that's now. At 4 months I could have maybe managed a morning or two.