if your planning on buying 1 second hand then you will end up having to replace the laser.Ive had my slim since november 2008 and the laser wearing out already and mine was brand new.mine is a SLim pS2 90002 and hasnt been taken apart or modded.My brother has a fat ps2(modded) that he's had for 6years (probably longer) and it works fine.My sisters slim model is about 2years old and works fine.
custom built gaming pc from early 2010,ps2 with 15 games all original,ps3 500gbs with 5 games all original,yamaha amp and 5.1channel surround sound speakers,46inch sony lcd smart tv.
Sony has created a new BIOS in the SCPH-9000x series PS2 models that negate any OSDSYS update using the DVD player hack.
Maybe you are lucky and can save the money for another singstar version you don't have until now.
If you want to soft mod the memory card with Free McBoot, then the easiest ps2 is the fat. Once you've modded it, you can install a hard drive and play games from it using hd loader.
I actually have a slim. My model number is 700012. I got it about when it came out, and it still works perfectly. I use Free McBoot on it with no problem.
The only real difference is that the fat has the option to install a hard drive, and the slim doesnt. If you just want to play your original games than you should get the newest ps2, simply because it is newer.