I think that depends on your level of expertise - what sort of maintenance are you capable of carrying out on your computer?

From a hardware point of view would you change a HDD? Would you add in more RAM?

From a software point of view would you know how to track errors in your operating system and find the solutions and apply them (usually from internet forums)?

If you're happy to do this sort of maintenance then I guess a second hand option is for you. If you'd like some sort of support then perhaps it is better to buy new. (Unless of course the second hand option also comes with good support - but as 300 USD I doubt if it will be all that good - I mean how many times is it worth their while to have you ringing them up?)

Don't get me wrong I think the tough book idea is great but I'm just not sure if it is right for you.