BCI is probably your best bet for a first, just based on size alone. Got my first about 3 years ago and havent had a problem so far. I got tagged once, but it was my fault, and he was young so it wasnt all that bad

. Tried to remove him too soon after feeding, whoops! I'd suggest getting a young boa from a good breeder. Might cost you more at first, but itll probably save you money in the end with potential vet visits and what not. Stay away from pet stores for the most part! We got some of the good guys on this site, so if you DO decide to get a boa, I'd ask around here first.
You might also want to look into the hogg island boas. Love their coloring and the stay on the smaller side.
GL with your deciscion!
Matt