You need a slim made previous to August 2011, if you get one now, it will not be modded till there is a new method released for both RGH and firmware mods. new motherboards and drives started appearing after August 2011.
If you find yourself a refurb you should get one that is moddable.
To mod slims you will either need to drill into the chip, cut a trace and solder and use a probe, or buy an xecuter replacement PCB board for the drive.
If you are scared of drilling or trace cutting then repairing with solder, you probably should just but the xecuter replacement PCB, desolder the old one and solder in the new one. this way you can keep your old PCB and not mess it up. Also make sure you get your firmware dummy.bin before you cut the trace or drill in case you brick.
jungleflasher.net might have a good tutorial.