How old is the baby we're talking about, 3 months?
The wattage of any bulb will depend heavily on what temps you want to reach and your home's/room's ambient temps. If you're not a heavy AC user then a smaller wattage will get you to the 80-84F, but if you're always running the AC very cold then you may need a higher wattage to get to the same temp.
I will say that I don't use more than a 60w halogen and I get mid-80 temps in my cages. So I would see what a 40w is doing and go from there. It may take a little experimenting or raising the basking light up a little off the top of the cage.
I would, yes, because he's going to be small and a little more delicate. You'll want to stay closer to 80F for a few weeks as his warmest spot.
Did you buy a pet store basking light? Those are usually a waste of money, as any normal incandescent bulb or halogen from a home improvement store is going to work just as well at producing light and heat. So if you spent $15 on a pet store basking light I would return it and just get a multi-pack of incandescent bulbs, for example, in different wattages for only a couple dollars. That way you can figure out which wattage is the best for you and you can still use the stronger bulbs later as he gets older.
Do you have a UVB bulb yet? These you do have to get online or at a pet store, normal bulbs will not produce UVB light.
Here is the forum's info section on chameleons, it might be helpful to look through it tonight (I know you'll be nervous waiting for the little guy anyway!) and it should help fill in any gaps: https://www.chameleonforums.com/care/chameleons/
I would say it all depends on how far you have bulb away from basking area. If you have a low watt bulb too far away from basking area/branch or vine you wont have too much heat in that area. That being said you dont want it too close to leave burns on your little one either. So I suggest you invest in a probe thermometer and wrap it around the vine or near basking area and go from there for what bulb is best for your setup. Keep in mind also having a uvb light source is a must have. I keep my temperature at 90-80 during the day and no lower than 75 at night using a 50w for my baby veiled that is about 3 months now and hes doing awesome.... hope this helps and good luck
Im from MA I have a stand for my light fixtures so I could raise it a little. I did get it at a pet store but because I ordered one on amazon and got another uvb bulb by accident so just exchanged it at petsmart.
For UVB I am using exo terra reptile uvb 100 26w. Its the tropical uvb one
I have a baby veiled around the same age as the one you're getting. I use a 50 right now, but the lamp fixture I have came with a dimmer that allows me to adjust it if the temp gets too high or low. The branches I have for his basking area are flexible, so I can also adjust the distance from the bulb with ease.