If you are planning to get a used one......check the lens thouroughly, and get a console with matrix modchip(plays games via IDE Hard disk)
If are getting new then its 5k+2k modding(I am not sure if matrix can implemented on ps2 slim but there are lots of modchip available out there including usb/swapmagic)
better get a 60GB PS3(plays ps2/ps3) and jailbreak.....no need to upgrade for 5 years again!