What does that 600 need to include? You said not RTR, so you already have radio and servo? Need to know what you need and what you don't need. Bearing in mind that you will need...
A servo (at least 200oz-in torque)
A speed control and motor
If you already have some of these, let me know, cos the more we can squeeze out of your budget the better. RTRs are rarely economical for someone like you who is willing to spend, as the electrics and radio are invariably crap.
Truggies are expensive beasts, up and running a good one will set you back a lot more than 700 with a good set of electrics. Just about the cheapest truggy option is a Hot Bodies D8T nitro kit, with a RC Monster D8T conversion. That will run you about 435 bucks and is well worth the money (but you still need electrics, charger, batteries, etc), it will handle incredible. However, you mention speeds of 60+. That's not what a truggy is for. It can be done, but if you want to go that fast AND have a capable off road car, without any compromise, you are better off going for a 1/8th buggy. Believe me a truggy at 50mph is MORE than fast enough.
Don't be put off though by the initial cost of the batteries and charger, you won't be replacing them for a long time and they will work for any electric car. That's always the problem with electric. Zero (almost) running costs, but high startup costs. The SECOND electric car someone buys, is invariably cheaper, as they already have the required infrastructure.
Another option (and one I wholeheartedly recommend) is to wait for the new edition of the Thunder Tiger MT4 G3, it will be 6S ready out of the box for 500 bucks and is a total madman. Should be out (again) soon.
The best value option for a buggy is the OFNA LX2e...
That's 440 bucks all done with everything except batteries and charger and will be an EPIC car.
Basically, whatever truggy or buggy you get, that ESC/Motor combo, servo and radio will suit it. The question then becomes how much you want to spend on the actual kit above the 250 bucks or so (plus charger and batteries) needed for the ancillaries.