Well with top down, you can use array based collision quite effectively. Meaming you would have a maze of sorts, flat walls and 90 degree corners. But the benefit is that its really easy to allow sliding collision, it is very very fast, and very very reliable.
I would advise using 3D models mostly, maybe characters can be billboard sprites, but buildings and stuff like that, really it's best to stick to 3D. For example, I could make a 3D building in a few minutes, but I've no idea how long it would take me to draw a 2D building sprite.
You could use point-to-point bullets, so they could be very fast without missing a collision, line to circle based collision should be plenty for your characters. If you imagine a little circle around the waist, and a circle under each shoulder, top down - a bullet hitting those circles is pretty damn accurate for character->bullet collision. Most 2D shooters will use lines, circles, but mostly cubes for bullet collision - so a little cluster of circles is probably better than most options. With the trajectory of your bullet, you form a line that can be checked with a circle for collision, there are a few examples of this about.
The main factor in these games is speed, things have to be smooth and fluid, or they are no fun at all - so it might seem like a step back to use array based collision, or line-circle collision, but actually that's what facilitates them, that's why they can be really fast paced and have lots of effects and enemies on screen.
For me, I'd probably like to make a game like that, fast paced and arcadey, lots of blood and guts, and even if your art is not 100% perfect, people will still get into it. Having thoutful touches here and there is more important than glorious visuals - it's an indi game, not a limp-wristed Unreal mod
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I would suggest buying the original DarkMatter model set - low poly characters, easy enough to break them up and make character meshes, lots of vehicles, lots of weapons. It has a lot of models that you would find usefull, just the right detail level for a modern top down shooter. With 3D models it's easy to change the textures, so having lots of enemies all looking different is possible. Also you might just use a 128x128 texture map per character, which should be fine for a top down game - on the other hand, if you did sprites then you couldn't fit very much in that space.
