Good choice on the basic SPG.
There are many good options for your first computer. I have not used the Mares, but here are the things I would require..
-Nitrox compatible.
As noted, all new ones are. But, watch out if you are buying used.
-user replacable battery
Definitely agree.
-Does not 'Lock out' if you go into deco mode, or miss a deco stop
I don't think this matters at all on a recreational computer. If a recreational diver skips a deco stop, it is probably in their best interest to stay out of the water for 24 hours. As far as locking out for going into deco, I don't know of any that do that. They just lock out if you go into deco and then skip a mandatory stop - which, as I said, I think is fine.
-can be set to gauge, or bottom timer mode (that way it will still be usefull if you upgrade to a better computer at some point)
Kind of nice to have, but not as big a deal as many seem to make it. It's only useful at all if you go into tech diving. Getting a better recreational computer means you can still use your original computer in "computer" mode, not needing gauge mode.
Nice, but not required:
*multi-gas and o2 up to 100% (if you ever decide to take more advanced training like adv nitrox / deco)
I think this is pure marketing fluff. Useless for recreational diving, since you never do gas switches or use O2 higher than 40% in recreational diving anyway. And if you go into tech, you're not going to be doing gas switches on a recreational computer anyway. Some people say it's good for pre-programming a few different gases and then quickly switching to the one you're using. But, my rec computer has this feature and I have never used it. It's just as easy to simply change the FO2 on Gas 1 each time I use a different gas. You HAVE to learn how to change the FO2 on your computer if you dive Nitrox. You never HAVE to learn how to do a gas switch on your computer. So, using the multi-gas thing just means learning one extra thing on your computer, that you don't even need. It MIGHT save you 5 seconds versus just changing the FO2.
*turn off audible alerts (much nicer for a back-up)
*bungee mount (vs strap)
*simple un-cluttered display thats not confusing
*I don't like air integration.. but some do.
I definitely do. Really experienced divers may have no use for it, since they know their SAC so well. But, that doesn't reduce the value of it to newer dives who can learn a lot about their SAC by reviewing air consumption data after a dive. Experienced divers might have no need for a buddy, either. Does that mean a new diver has no need for a buddy?