Hi Affirm! Congratulations on your new baby.
A lot of people will recommend some of Cathe's older step tapes because the moves are easier to pick up, but I say try some of her newer ones. (I prefer not to workout to dated moves, music, hairstyles, and clothing).
Since you said you are giving yourself 2 months to get back into exercising before you start with Cathe, you should be good and ready for some challenge by them. I recommend Rhythmic Step and Interval Max if you just want a step work out. Rhythmic Step is Cathe's newest all step tape and has some complex moves, but it fun and is closer to intermediate level than advanced.
Interval Max is a challenging interval workout, using mostly the step, but the moves are easy to follow. During the intervals you can you as little or as much power as you feel up to. It is definitely a workout that will grow with you and continue to challenge as you master it.
If you want a floor and step tape that is not too complex, try Maximum Intensity Cardio. It is long and tough, but will definitely get you to your weight loss goals.
If you want more bang for your buck you can get videos that combine strength training with cardio. My favorites are Circuit Max, Body Max, and the best for those in a time crunch, Cross Train Xpress. If you have the money, I definitely recommend this series as a great first purchase. You get so much, including 5 cardio workouts, a ten minute segment for each upper body part (which can be done in one 50 minute section if you buy the DVD- another reason to buy this one!), several (4, I think)ab/plank routines, an entire lower body strength training workout of 40 plus minutes, and they are all arranged into a 6-day rotation. (DVD is most convenient for mixing and matching sections).
For all strength training I recommend Power Hour if you want to workout all muscles groups in one hour long workout. If you want to break it up by day then again consider Cross Train Xpress, or try the Pure Strength Series.
As for your abs, just consider them another muscle group you want to tone, as you would your legs or biceps for example. Decide what you are comfortable with and commit it into your exercise routine. I'm not sure how many days would be best for you. However, I prefer to do them every other day as I do with my other muscles groups, because building/strengthing muscle requires rest days in between. The planks in Cross Train Xpress and Circuit Max will definitely challenge you core/ab area.
Hope this helps some!
Emily