Got me an ofna buggy

[Batfish]

5000 will probably be fine in the center and rear, but your steering may suffer a little with that lube in the front. If you haven't driven 8th scale before you may not even notice a difference, though. Like you said, use what you have and work from there :)
With enough voltage and high enough gearing, I'm sure you can get to 60mph. You had better make sure you ease into the throttle, though. Once you're geared for that speed any sudden throttle movements are likely to push your setup too far, which could be a very bad thing for the wallet ;)
If you plan on racing the buggy regularly, check the speeds of the buggies at the track and just try to match their speeds. They're already running as fast as the track will allow so there's no need to make yours 10mph faster. If you're just going to bash with it, gear for your maximum fun in the area you run it :)