Just to be clear, you have a Kenwood TS-450S that doesn't have the automatic tuner already built in? You didn't mention this specifically in your original post. The internal tuner is/was optional, but most of the ones I've seen on the used market had the tuner included.
If that's the case, then that's a shame: the built-in auto tuner is pretty nice, and it should be able to handle a G5RV without any problems. That's if you intend to operate barefoot though. With an amp, you'll still need a separate tuner.
I happen to have the MFJ-962D. It works fine, though in retrospect I wish I had gotten one with an internal dummy load.
There's one small thing I would watch out for if you choose to get it. The MFJ-962D has a light bulb behind the meter on the front panel, which can be powered by +12v. A small power cable is included that plugs into a jack on the back of the tuner. You can wire this to the back of the power supply you're using for the TS-450. Pressing the red power button under the meter will light it up. (Alternatively, you can buy a separate 'wall wart' power module.)
I have a TS-850S/AT and a PS-52 power supply. For some reason, when I had the power cable connected to the PS-52 to light up the meter, I kept getting RF feedback in my audio on some bands, particularly 40 meters. As soon as I unplugged the power from the back of the tuner, the feedback went away. I suspect that I created a ground loop somehow. In any case, I didn't have a burning need to keep the meter illuminated, so I never bothered reconnecting the power cable. (You do not need it for the peak reading function on the wattmeter.)
This may not occur in your setup, but if you do notice an RF squeal in your audio and you have the light plugged in, try disconnecting it at some point to see if it helps.
I have to admit that I've been spoiled by the auto tuner in the TS-850. It's really very quick (enable it, then just press the tune button and it typically locks in in just a couple of seconds), and it remembers settings when you move back and forth from one VFO to another or between memory channels. I tend to jump around between 10, 15, 17, 20 and 40 meters a lot, so this is handy. I've never looked into getting an automatic external tuner though. They're more expensive, and it's one more thing you need power for. If you're on a tight budget, the manual tuner is probably better: just keep some notes of where the 'sweet spots' are on your favorite bands so you can dial it in quickly as you move around.
-Bill