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Since you state you are somewhat budget constrained, buying a used console on eBay sounds like a good idea. Take the SPG/computer and compass out of the console and fit the last two on separate wrist mounts. AI computer vs SPG? If you don't pay attention when diving neither will help you when you run low on air.
Nothing wrong with that approach. I bring an SPG in my save a dive kit, but don't generally bring it on the dive. However, a lot of that will depend on how you dive. I usually dive private boats with some friends. So, worst case, we surface and I swap it out to continue. If doing charters, that may not be optimal, though so I could see the benefit of bringing the SPD along.