-Where I buy: This depend how big I estimate will be the chance I need to warranty the item. For certain items, like LCD displays, the chance is big and it's not easy to send massive 20kg box and pay for courier to abroad. For this reason such items I buy locally. I have this kind of e-shop here which also has massive drop-off brick&stone sort of shop. So when I need to quickly warranty something, I just drive 5 minutes by car and do it personally. This often costs me certain premium. If comfort of mind is important for you, I would buy everything locally this way.
-If I want to get something cheaper, or the item is obscure or niche, I shop at e-shop aggregators like Geizhals.eu. I needed specific memory set and I didn't want to pay 900 for it at local e-shops, so I found where I can get it for 700 from Germany.
-I do shop a lot from Amazon too but only if the price is right. These are often the same e-shops as above, but with added security layer from Amazon.
-Waiting right now might be good idea. This was rushed start and majority of items are simply not available. Motherboards, CPU coolers are very sparse. The latest nVidias will likewise release next month and following.
-You can add up to four GPUs for this platform. But this is only if you decide to go GPU-rendering, for example Vray-GPU. Vray does seem to have a lot of stuff prepared for the upcoming generation to get as much potential from the latest card as possible.
-Corona currently uses GPU only for single task, instant denoising for previews by nVidia Optix algorithm.
-When it comes to all those brands and models for cards, I don't think anyone can answer ultimately which one is the best :- ). For me, it's the cheapest you buy. No reason to pay for fancy heatsinks and tiny factory overclocks. I don't have any particular favourite brand or model.
-If I had money to burn, I would buy most memory if I were to stick with Corona. Corona is super memory hungry, and if I want to keep opened two scenes at same time, bunch of big high-res Photoshops, run simulations, photogrammetry, etc.. memory is invaluable.
-Water loop will give you additional 10perc. of perfomance and if you go for the big one (double radiator), then both 10perc. more performance as well as very silent operation. This is good idea only if you plan to stick with CPU renderer like Corona. Otherwise it's wasted.
-Difference between current 1080ti (ideal to buy at 500 second hand or 600 brand new euro) vs 2080 at 800 euro vs 2080ti at 1200+ euro is how much you care for real-time tasks. If you outside of Corona rendering also do a lot of Unreal Engine 4, or plan to try some of the GPU rendering in future, it's better to go for the best.
Otherwise stick to lower model as softwares like 3dsMax, Adobe Suite (photoshop, lightroom, premiere,etc..) use very little and ineffectively GPU power. But don't go lower than 1080ti at this moment as I found out lot of softwares with viewports (like 3dsMax) start to struggle if you run multiple high-res displays (I run 3x4k panels).
