Flash is selling his Fat PS2, just pointing it out.. he will probably negotiate with the price listed.
http://gbatemp.net/t258351-wts-ps2-bundle
But buying slim or fat is your decision. I like the slim myself (smaller, quieter,.. well thats about it i guess lol, they are also known for using cheap lasers that don't read disc well, but you can replace them for around 10$-20$), but also like the original for its hdd support (but since there so old, it's hard to find a reliable one with a good laser). 90001 will only works with modchip.
Modchips are the way to go in my opinion if you are running off disc. It works without needing to patch iso, less compatibility issues than esr (I have never tried open ps2 loader or what ever it is called though so I have no idea about compatibility using that program), able to run ps1 and ps2, multi-disc support with ease. But there are cons too, its not a memory card mod so its not portable that you can bring along with you, certain ones still need memory card to install programs on it (that is if you want programs). Cost is a con also.
Oh yeah, try to avoid V9, V10 and V12 (scph-70012)
They have some problems with laser burning up. They have chips/solutions that claims to prevent it though.