i wouldnt use an aquarium for a boa. they are hard to get to hold humidity and also to hold ambient temps. if thats all you have, yes it would work and has for a long time. but its far better to use a wood or pvc style cage. or even when its young to use a tub would be best in some opinions.
if your buying a baby/young boa you dont need to have a real big enclosure for it right away. so a tub could work and you could save up for a built cage or build your own if your hand enough.
a few things to look into are a good thermostat, heating sources, flexwatt, heat pads, bulbs, ches, or radiant heat panels. what style you want to use and how your gonna set it up.
and then figure out bedding and cage furnishings, hides, bowls, foliage.
and it never hurts since you have time to look around at all the good breeders. really nice snakes are going for a lot less than they used to. i know i cant wait for my new boaphile to come in so i can get another.