Great questions about water cooling your GPU! I can offer some guidance based on my experience.

Regarding your first question - whether to buy a GPU with an integrated AIO vs. installing a custom water block yourself:

Pros of buying a GPU with integrated AIO:

• Easier, plug-and-play setup
• Don't need to worry about compatibility between the block and GPU
• Often cheaper than buying components separately
• Warranty usually covers the whole unit

Cons of integrated AIO GPU:

• Fewer options, you're limited to GPUs that manufacturers make with AIOs
• Can't customize the look or performance as much
• If the AIO fails, whole GPU needs to be replaced or serviced

Pros of custom water block:

• Can choose from many GPU models and blocks for your desired look/performance
• Easier to maintain and replace just the block if needed
• Better cooling if you have a custom loop with good components

Cons of custom water block:

• More effort to research compatibility and install it yourself
• Higher cost when you add up GPU + block + other loop components
• Installing the block likely voids GPU warranty

Regarding your second question - in most cases, it's not recommended to try connecting a GPU block to an existing CPU AIO. They aren't really designed to be expanded like that. The pump and radiator may not have enough flow and capacity to handle the extra heat from the GPU. It's better to either use a GPU with an integrated AIO, or set up a full custom loop for the GPU.

Some good resources to research GPU block compatibility:

• EK Cooling Configurator
• Alphacool GPU compatibility chart
• Watercool.de, Aquacomputer, Bitspower, etc. - check block product pages

I'd recommend watching some tutorials from Jayztwocents and other PC water cooling channels to get an idea of what's involved before diving in. Definitely plan out your parts list carefully.