A post...for me? I'm honoured!!
1. You need to take the dashboard out to get to the heater unit out so it can be split to insert the evaporator. The radiator needs to come out to get the condesor unit in front of it. Once those two are in & the compressor on top of the engine, it's only three rigid (aluminium) and two flexible pipes to connect it all together.
The wiring you will probably find (as I did) is half there, half not. I had to make a partial loom section to get the compressor relay & compressor connections all together. All the wiring for the condensor fans was already in place. I went to the trouble of getting the right coloured wires & making the loom as 'original' so at least if it ever goes wrong, it should be easier to trace. If you take on the job, I can score you a few of the male multiplug connectors, cos I needed about 6, but had to buy a bag of 100 from the distributor.
I did it all on my own, the only physically awkward bit being getting the dashboard out & then back in again.
It took me almost 3 months in the end, mostly 'cos I knacked my back up while trying to find the right wiring, lying in a stupid position under the steering column! If you had all the bits to hand, it could be done in a weekend. The main problem occurs when you need a bit & it's 18:00 on Saturday, so you're stuck until Monday!
2. I got a complete setup off a rolled '96 Tdi manual for £150. Kept putting adverts on every forum classified section I could find. Got lucky on LRO in the end. I've heard of kits being sold off cheap every now & again, but never seen one. Definatley a better idea to get the whole system off one vehicle that same as yours, as there are bound to be slight differences year to year. The V8 & TDI compressors are different, and not interchangeable. The wiring is also different between V8/TDI/auto/manual. TDI manual is the easiest!
3. If getting a second hand one, it would be easier to take it off the vehicle yourself, so you know how it goes back, and get all the bits. Get all the wiring you can (the short loom section that attaches to the heater unit is different & the section that connects the heater/fan/AC controls is too). The AC relay is behind the front R/H kick panel, but the wiring may well be attached to the main loom & be a sod to get out. Don't forget the idler & tensioner pulleys for the compressur drive belt.
The best way to deal with the wiring is to print off the relevant section (well, preferably all) of the wiring book from the RAVE manual, acheive a Zen-like trance & understand the wiring eventually you shall, young Jedi.
I left off the special thermostat elbow peice with the two temp switches in it, as they seem fairly unnecessary for a TDI in the UK. All they do is turn on the condensor fans if the engine gets too hot (A/C on or off) and turn off the AC if the engine gets too hot.
You'll probably find as you take the radiator out that all the fins fall off & you'll need a new one. And all the bearings for the three drive belt tensioners & pulleys need replacing (but they're only £2 each form a bike shop). Might as well fit new drive belts, radiator hoses, thermostat & anything else that might go 'pop' one day while it's all in bits. And clean the intercooler while it's out (mine was almost completely blocked on the outside with goo), 'cos withe the air con condensor in the way of it, it needs to be as efficient as possible.
Make sure if it's a second hand setup to clean all the pipes & connectors properly & DO NOT let any muck at all get into the system. Find a local friendly air con shop/chap & get a set of new O rings (there's 8 I think) for the pipes, a new receiver/dryer fitted (much easier before the condensor unit is fitted) and the compressor refilled with the correct ammount of fresh oil.
If you get all that done, then fit the system (and convince yourself that all the wiring works) it's about 10 minutes work to get it vacuumed down & gassed up. Then you'll be so cold a new wooly hat will be needed.
All in all, it's a fairly easy job, as long as you're confident taking your motor to bits & rebuilding it. The actual AC system is very simple & even the wiring is very straightforward (once you understand the system & find which bits are there and which need making).
Any questions, comments/bits I've forgotten, let me know.