If you're on a tight budget, then consider a T-line instead of a reservoir. Do a search, there is lots of info about how to do a T-line.
The garden pond pumps you are probably talking about are like the Eheim 1048 or it's bigger brother Eheim 1250. There are others, but those are known to be excellent pumps. I have an Eheim 1048 and love it. But, it's 110/120v AC, so you need a relay and outlet which adds expense and more modding, not to mention they are fairly large pumps. For your purposes, you'll probably want a 12v DC pump. I'm out of the loop in regard to pumps, so maybe others can help.
I would think a Swiftech MCP350 would work good for you paired up with a Swiftech MCW6000 water block for socket 775 (if this is for your main siggy rig). If you shop around, that will probably cost about $110 for the two.
Next is the radiator. I prefer 2X120mm rads for good cooling, but if you have a small case and/or want to save a little, then a dual pass single 120mm rad will probably be ok, like the Swiftech MCR120 for about $30 or maybe better yet the Black Ice Xtreme for $40. If you go with a car heater core, you can probably save a little. Do a search for what people are using. Generally a new one at Autozone will run $20 to $25.
Now you need tubing, fittings, and clamps. Your pump and rad will probably be 3/8" ID (1/2" OD) and that's probably the cheaper route. Don't forget a little anti-freeze and distilled water. That should be well under $175 with shipping or less if you find some good bargains (and go with the car rad).