It depends on where you buy it.. a brand new model PS3 will probably not only have the lates FW (3.70+), but also be the new 'so called' un-hackable consoles so I'd only buy one of those if you want to go entirely legit..
Other stores might have slightly older models but they will still come with FW 3.56+ - which to date is still unable to be modified to run CFW, homebrew or backups.
Best bet is to dig around pawnbrokers and the like for a earlier model console. Just get them to boot it up and check the fw before you buy..
Even if you buy one with later FW installed, if it has it's original HDD still in it (you'll know if it's ~120GB or less), there may be a way to re-flash back to it's original FW with hardware (eg like Progskeet). Read up on this before you buy as you will have to buy a NAND flasher first and then know how to solder (quite skilfully), to do this yourself or know someone who might be able to do it for you.
"We all agree your theory is crazy, but is it crazy enough?" - Neils Bohr