Naturally it depends upon your content and end usage. However, purists will tell you that, at the very least, you need 4:2:2 colour sub-sampling to pull a clean key. Don't believe them.
For talking heads, we pull a clean key from a 4:2:0 Sony PMW EX1R.
Good lighting goes a long way to help: making sure that the subject is sufficiently far away from a beautifully evenly lit green screen. We throw a complementary magenta rim/halo light on the back of the subject's head to offset potential spill. Also, very soft lights at 45 degrees on the subject creates a good shadow-free image.
Probably the best option for your budget would be an external recorder like Atomos Ninja Star, which you can connect to your Panasonic camcorder via HDMI port. You can obviously have a look at Blackmagic Pocket Cinema Camera for £635, but that's just a body, while you need at least fast enough SD card, a lens and an external microphone (if you need audio) as well.
Consumer cameras usually shoot AVCHD video. This is a highly compressed format and the limited color space and intra-frame compression can make pulling a decent key very difficult. Frankly, I've had better success pulling a key from a iPhone 5 or 5S than from a lot of AVCHD footage.
If your cameras lens quality is very good and you have HDMI output picking up video recorder might be an option. Blackmagic Design's Ultra Studio Thunderbolt Mini is a good option at about $150 US. Blackmagic Design: UltraStudio Thunderbolt™
If you want a new camera and can afford it the Blackmagic Design's Pocket Cinema Camera at $1000 US stands alone for price/quality. You will have to pay twice that for any new camera from any manufacturer that has the same capabilities. Lenses are not very expensive. You can pick up a fixed focal length or small consumer zoom for just a few bucks.Blackmagic Design: Blackmagic Pocket Cinema Camera
Find yourself a used Panasonic HVX 200 camera. Make sure it comes with P2 cards. This camera shoots in 4-2-2 color resolution, and it does NOT use interframe compression.... both of which will greatly improve on any itty-bitty consumer camera.
Now, if you don't use the proper lighting technique for chroma key, there isn't ANYTHING that will help you pull a good key. Keep that in mind.