Honestly? Just get a plain tank. That way you can customize all the equipment to the betta's needs instead of ending up with a kit intended for goldfish
I agree whole-heartedly. I am still a fish newbie, but let me share with you what I've learned so far. I bought a betta before really considering living conditions. I bought it a small 1.5 gallon tank, because the pets stores lead you to believe that is humane. I mean, they sell the tanks, right? They keep the fish in a small cup. Can't be bad, right?
I didn't research until AFTER I had him for awhile and he was looking sick. After being scolded by a few Betta enthusiasts about anything less than 5g being inhumane, I bought him a 5.5 gallon pre-packaged special. I ended up having to replace the HOB filter, and the heater, and now, I can't even really find a lid to fit the tank I bought. After all of that, I spent more money than if I would have just bought each piece separately.
If I could do it all over again, I would invest in a 10 gallon glass tank (because they are cheaper than the 5 gallon ones here in the States, but they DO sell 5 gallon glass tanks), buy my AquaClear/Fluval 30 gallon HOB filter (water rate is adjustable and you can "baffle" the water output with a sponge. Betta don't like the strong current. If you rubber band a filter sponge on the waterfall, it reduces that), and a good 50w heater. You'll also need a glass cover for it, since betta jump. If you plan on putting real plants (which I HIGHLY recommend), I'd research your lighting options. In the end, you'll be happier, and so will your fish!
Needless to say, I am upgrading Deathwing's tank to 10 gallons in a few months when I have time. He's my at-work fish, so it is harder to set up new tanks, but it is going to be done.