I have an 89 305 LO3 Firebird. It has a 1228746 ECM with 160 Baud output. Will it do me any good to get a scanner for this combination? What one should I get?

Um, unless you're programming your own PROMs on a regular basis, or your car is well on it's way toward earning now sensors, I don't really believe a scanner would be worth it for you at this point. If you're curious about the running condition of your car, such as whether or not it's trimming or adding fuel in long term (BLM) or short term (INT), then a scanner would be the way to do that. It's just up to you on how much money you have sitting around..

I have an AutoXRay currently, and will also be picking up a Diacom later on down the road whenever I can find a power cable for my ancient laptop.

I don't regret buying the AutoXray, per se, but for now (since my cars are running so great) it's not very useful, and especially on my 89 RS 305 TBI. The information is almost non-existant: about 23 data points with a horribly slow data rate. I hooked it up to a 95 Vortec Blazer a while back, and was blown away by the speed and the number of data points that sucker put out! Something like 45 different types of data updating numbers almost too fast to read.

I am going to program new chips after adding some parts. The question is - will the ECM and low baud rate give me problems later. Should I change it before doing anything else?

Don't worry about the baud rate. It's slow compared to when you connect it to say a 93Z28 but it's very good to have to see what the engine needs for tuning chips. Good luck.