[rigspelt]

It's not technically difficult for a careful DIYr with decent wrench skills and sound basic understand of engine function. Excellent information is scattered through several threads in the forum archives, and well worth locating with the search tool.

Planning is important. Finding places around the engine for the heat exchanger, reservoir, additional hoses and second pump needs to be considered before purchasing a kit. There are two main choices: MMI's second pump running off the front of the engine, and Indigo's second pump running off the accessory drive. Both kits use the same heat exchanger, I think. The raw water hoses feeding the heat exchanger are a larger ID than the stock A4 cooling hoses feeding the engine block, which might require a different tailpipe on the raw water intake seacock, and a different nipple on the discharge side in the exhaust. Buy good quality hoses. There was a very good thread recently discussing the relative heights of the heat exchanger, engine and reservoir.

Execution is mainly a matter of sound plumbing and wrenching practice.

Feel free to ask questions. There is a lot of talent in this forum, and a lot of ways to do this.

When I was muddling through this, I made the attached image. Buyer beware: I am a one-off amateur DIYr, not an expert.