The only thing you have to watch out with hard disks is that they align properly with the PS2 HD adapter. My Western Digital 120GB did not align perfectly, and I had to make-do by cutting the thin metal cover of the PS2 HDD adapter to align its molex with the molex on my hard disk. I do believe that most Maxtors are "compatible" in this area; but it's safest to check before you buy. How big is totally up to you, since the newer versions support 48bit LBA and can read past 137GB; I simply delegated an aging hard disk to use in my PS2.
Read up on the Independence Exploit, it's the most convenient way of launching HDLoader amongst other useful applications. As for installing games, get a cheap IDE enclosure and use WinHIIP; it's insanely fast and easy to use. Or you can use HDL_dump/dumb to transfer the image over the network, which is slower, but is less hassle as you just need to plug in an ethernet cable. The latest version is compatible with DHCP, so as long as your router/switch has that on, it's just plug-and-go.
Some games that have issues can be resolved by using the three HDLoader modes; others require certain patches to work while a very few games won't work at all. Look at the
PS2HD compatibility list to see if games are compatible and which modes need to be enabled.
HDLoader is definitely worth it. The convenience of having all your games together and the almost nonexistant load times is worthwhile. Only gripe I have with HDLoader is that when I reset to play a different game I can't stop the hard disk from powering down and back up again. This may wear down the disk faster than if it was in a PC, but they're so cheap noadays it's no longer a worry. (Plus mine's already old and worn so..!)
Jam it back in, in the dark.
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