Actually, building a great tank can cost as much or more than buying a pre-made. It all depends on what you want for your snake, and what your snake prefers.

Both my boa and my ball were rescues and rehabs that I have kept (horrible, aweful original owners).

Sage, my red-tail (is a true red ;) ) prefers to curl in the bottom corner of the tank, so shallow for him is best. Use the tank you have and offer high and low places and see which yours prefers. Then when you build a tank, you can customize it to fit your snakes preference.
Sage also hates hiding places and avoids them at all costs. He was that way when I rescued him.
When I worked at the Herp store all the other boas like hiding places, so definately offer one.
He will be 4 this summer and is about 5 1/2 feet and a hearty eater.
He is trained to know when the tank is cleared and the white bags appear that he is getting fed.
Because he is a rehab his demeanor is not as docile as other Red's I have had in the past, but he is still calm...just not good in groups of people. All of the other boas I have had or rehabbed (7 of them) have been wonderful to have.
However, I would reconcile myself with the fact that your boa *will* be larger than your ball, even at it's smallest growth a red is, in general, larger than a ball at it's largest growth. There are always exceptions but it is rare.

For climbing, bleached, solid driftwood is awesome. There are not a whole lot of splinters to worry about. Anything you bring home you should bake in the oven at 275 for at least an hour. It was a trick I learned at the store. We soaked rocks, plastic/clay tank materails in a diluted bleach solution and then ran them through the dishwasher without soap on the heat dry cycle.

I use terry cloth towels as a substrate. I run them through the wash with bleach, then wash it in just hot water, then dry it on high. Fully sanitary and reusable.

I have never had an issue with Goldie (my ball) eating. Even when I first rescued her. She is actually one of the most amazing ball's I have ever seen/owned. She is so docile there are times she doesn't even squeeze her food, just gulp it down.

If you purchase a Boa from a pet store more than likely what you will be getting is the BCI, or common boa. They are by far more common in the pet trade then any other subspecies of Boa Constrictor. Sometimes they will call these Red Tails or Columbian Red Tails.

People who sell "True Red Tails" will usually refer to them that way occasionaly using a locality name like Surinam Red Tail and or BCC (Boa Constrictor Constrictor) and they usually charge quite a bit more for them.

Beyond that they are extremely difficult to tell apart by just looking at them and since the ranges of the two snakes overlap and they can and do interbreed there exist many intergrades.

In general the BCC is a bit more difficult to keep then the BCI. The BCC is quite a bit bigger and some feeding problems may exist.