no way is a 1/5 scale going to do 65moh+ unless you've got a football field to run it on and a dump truck full of money to throw at it. i think 50 is a stretch for those things. then again a nitro monster truck typically won't go 65+ ether unless you heavily modify it. electric monster truck might do it with a 6s lipo battery and some gearing. it's pretty easy to get an 1/8 electric buggy or truggy to do it.
there's also a trade off between speed and functionality. high speed, high gearing, high heat, lower run times. you also have to set the chassis up to handle at high speeds which means it won't handle turns or jumps very well. you sacrifice functionally for speed.

realistically you do most of your bashing/driving between 10 and 35mph no matter what type or scale RC you've got.
you need to evaluate how you're going to be using the thing. where are you going to drive it? how are you going to drive it? how much space do you have? what kind of terrain are you driving on? is noise going to be an issue? how often are you going to drive it? where can you get parts? how much wrench time and maintenance are you willing put into it?