Welcome to the forums.
I'm not a pro or anything, but I can help out some.
The RBX and chevette would be a good combination.
For a pump, you might want to check out the MCP600. I've heard good things about it and I'll probably be getting it for my next project. You might also look into the Eheim 1250. They would probably both be enough for your setup. The Eheim would be quieter, but the MCP600 can be connected to a 12V rail from your power supply. I'd get the MCP600, but that's just me, they're both really good pumps.
I'm not sure if the heater core would fit, but you could mount it anywhere. I think it should fit, but I'm not 100% sure. Try to mount it so that it serves as an intake, sucking cold air into the case. If you mount it on top as an exhaust, your temps might suffer a little bit because all the warm air rasies to the top and it's around the psu and cpu area which get hot.
And yeah, you could always add a second heater core if you want to. You just have to find a way to mount it.