I think you should go with a 25 amp controller because that allows you to buy almost any battery you want so you'll be able to shop around for a good battery rather than be limited to only High C rate discharge batteries (which are more expensive). You can still go wtih a 40 amp controller I guess but then you have to find a 2C battery that actually works well with a 40 amp discharge rate. And some of them that say 2C are not really 2C batteries. I have Thundersky that I use with my 35 amp controller and it just barely can handle it. I'd go with the 2806 Nine continent. Do they still sell the 2805 nine continent? That one really is built for speed. The 2806 is kind of mix between speed and hill climbing. I wouldn't go with the 2807. Either the 2806 or 2805 Nine continent. Or a crystalyte hub motor.
Ok, if you go with a 25 amp controller, for a battery, it depends on what you want. Do you mind how big the battery is? Do you want something really small? Those are questions you need to answer. I would not buy a battery off of ebay.
For batteries, if you go with 25 amp controller, you can get any of these:
pingbattery ...from pingbattery.com
Thundersky from elitepowersolutions.com (that's where I got mine...but they are a bit big but they are still good lithium batteries)
lithium batteries from hightekbikes.com
lithium and NIMH from ebikes.ca
cycle9.com has some lithium but usually more expensive
itselectric.ca has some but usually more expensive
If you want a small battery then Thundersky are probably not what you want. They are smaller and lighter than SLA because it's lifepo4 but they are prismatic cells and they are very user friendly but they are kind of bulky but are very safe. They have a vent at the top to prevent fires. They are probably the safest lithium battery on the market. You probably want to go with lifepo4 since it's pretty safe. Careful because there is a difference between LIPO or lithium polymer and LIFEPO4. I hear that LIPO has gotten safer though but lifepo4 is still safer than LIPO. LIPO gives higher C rate, though.