D.W.
I started cloth diapering my daughter when she was 4 months old and it has been a great decision. The only time I have to buy diapers is if someone is baby-sitting and they don't want to use the cloth. Talk about savings. She has been using the same diaper for 16 months now and she is already potty training! I think it's because with the cloth she can really feel when she goes. Cloth is definitely different from disposables but well worth the learning curve! I really like the Bum Genius one size pockets. What I would suggest is getting one of a couple of different brands to see how you like the fit on him. Thirsties brand also makes a one-size diaper that I think would get your farther than most. We used the larger Thristies pocket diapers on my son at night when he was older rather than using pull-ups and thristies go up to 40lbs. I think your son is right at the weight for him to start right in their step 2 so you wouldn't have to buy another size! Also you can check out reviews for any diapers you are interested in on diaperpin.com. Pinstripesandpolkadots.com has great info for a newbie and the lady who owns it is super nice and would gladly answer any questions you might encounter before you buy. If I had it to do over I would have gone strictly with one-size products.
You could probably get away with 15-18 if you were going to wash every other night (which I think is ideal). You will change your baby about every 2-3 hours in cloth depending on how heavily he wets. Also invest in some hemp liners for nap and nighttime. I use 2 hemp liners along with the insert that comes with my BG diaper for nighttime. Hemp is very thin but very absorbent.
I use Arm & Hammer Essentials Free to wash with. And the great thing is you only use about 1 tblsp of detergent per load!
I highly recommend getting diaper liners that can be flushed (Bummis or Imse Vese makes them) They are fantastic for when the little one starts solids and poos. Just lift the liner out and flush. They are biodegradable! So no dunking the diappies. Also, if your baby is strictly breastfed right now you don't even have to rinse the poopy out. It washes right out in the laundry. You do a cold wash/rinse, then a hot wash/ rinse and another hot rinse and toss 'em in the dryer! It is so nice not to have to constantly spend money on diapers!