I'm no expert, but I'll try to take the questions in order.
1. I prefer pump. It's not significantly slower than an autoloader, it's cheaper, simpler (hence probably more reliable), and lastly, the racking noise of chambering a shell will send all but the most determined BGs away in a hurry!
2. Finish depends on the kind of abuse you expect to give it. I have a blued one and it's held up pretty well as long as I keep it wiped down once in a while. Stainless 870s are good, but a little more pricey. Others could probably answer this one better.
3. Go with the regular stock. If you ever shoot 3" shells in it, you'll know why.
4. Sights, again depends on what you want to do with it. A bead will serve fine for HD, but if you ever plan to hunt anyhting other than dove with it, some rifle sights or a ghost ring couldn't hurt.
5. As much mag capacity as you can get w/o making the weapon cumbersome.
6. I'd go with 12, but that's just me.
7. A sidesaddle should be okay as long as it doesn't interfere with the operation of the gun and its controls. Same with a light, just remember the light points both ways!
8. Mossberg 500 series, Remington 870 and Win1300 are all good pumpguns. You'll probably get a lot of votes for the 870. It's a keeper. The other two tend to be a little cheaper.
9. Shot size depends on where you live. If it's an apartment, then #4 would probably be about the maximum since you don't want it to sail thru the wall and ruin your neighbor's day. At indoor HD ranges, in all honesty, #7 1/2 would do fine and make a mess. If it's a house with close neighbors, you might get away with 0 buck, but who knows? If you're all by yourself in the country, by all means go for a mix of slugs/00 buck. I'll defer to the experts on this one.
10. You'll probably never have to replace an 870, but I disremember the cost of one right now. (~$350?) The other two can be had for around $250 for a no-frills package (18" barrel, 7rd mag, blued, and wood or synthetic stock). Used is of course cheaper and you can check the gun shows, but that sweet deal you get may have been run over by a truck at some point. New ones are so cheap that people tend to beat the heck out of these types of guns. Nice thing about a shotgun, is you can swap barrels to go from a wingshooting config straight to HD. Winchester sells a package like this for the 1300 I believe.