Best is what Jann suggests. I hear that adults can take a drop into the 40's but I don't want to try it out (in the wild they can obviously survive big drops but we want them to be comfortable, right?)
I use 70F for babies and 60F for adults as the minimum I allow.
Here is a great all-round caresheet for Veileds - https://www.chameleonforums.com/blogs/chameleonsinmyhouse/395-veiled-chameleon-care-sheet.html
Do temperatures drop below 60 in your house? That would be unusual if so. Unless they do you do not need a heat lamp at night, and in fact a night light will disrupt their sleep (even a black or red bulb).
it depends on your house temps or where ever you are keeping them. A 25 watt might work in my house but maybe not yours. The only way to see is to put the heat in there and measure the temp. Also, you know not to use any lights at night. Just heat. So you would have to get a little ceramic heater.