Indeed, find yourself a used TI-68k or early (Clickpad/Touchpad) Nspire, at $70 or less
Nspires are less programmable (some math programs haven't been converted to the Nspire, and assembly math programs are not usable but there are relatively few of them), the OS has slowed down so it's not as fast as the difference in raw CPU power between the Nspire and the TI-68k series could make us hope for, and I hate both Nspire keyboard types (even though I have long, thin fingers !) - but clearly, Nspires have more storage space.

Get both of them if you're allowed two You can never have too many calcs.

I passed the BC part last year (with a 5) using a CX CAS. The people that had 89's did fine too, but the people with 83/84's were definitely envious...

I can't imagine using two calcs at the same time during a test... one should be fine. As to whether or not you should get one with a CAS, I don't know.

The TI-89 is a great choice. I prefer the earlier models over the Titanic -- errr I mean Titanium.

I personally have two TI-89's(non titanium). They're both wonderful!
I still now and then try people who want to buy a new calculator to get this one, although I must admit, that the 89 community has kinda died, an d it doesn't really look like anything is going to boost is right-away up.

I don't think it is necessary to go and buy a CX CAS rightawy for what you have to do, but I can understand that it would also be a nice calc to have. But in my opinion, the TI-89 is for sure also a must in a collection.
I had luck and bough both my 89's second hand(the one was bytheway still looking brand new) for less than 30 euro's each(shipping included).
I can't say for you, but it may be a lot easier to find a cheap 89 hanging around than to find a nspire cas.
Also, I have to say it, I know the nspire's now have lua, but the TI-89 has a good TI-basic language as well, which ought to count for something.
I once wrote this short article, you might want to read it: http://www.omnimaga.org/index.php?action=articles;sa=view;article=92

Hope you're happy with your decision , whatever it may be.

$52 with shipping and SilverLink, I hope ? Without SilverLink, it's a bit much.