Basically, I would always use the Eiszapfen Pro fitting. Due to the fat and conical O-ring, these are the easiest to handle and the safest in terms of tensile force. This is many times higher than with the normal HardTube fittings. But unfortunately they also cost more.
I personally am not a fan of PETG, I would always use acrylic tubes. PETG is quite soft and easier to bend, but it also has disadvantages. Because they are softer, they bend even at quite low temperatures, which can lead to problems at connections such as the graphics card (not necessary, rarely happens). They are also clearer and do not stain. However, acrylic tubes are also more difficult to bend.
The basic kit is sufficient if you still have a heat gun with temperature control. This is important as the pipes are heated rather slowly and at lower temperatures. Too hot and you get bubbles.
It is important that you see what the structure should look like. Ideally, you need as few bends as possible. Various angle adapters or off-set fittings are usually used to compensate for minor height differences.
But all in all, it's not rocket science. You just need a lot of patience and time. The problem is, without knowing or seeing your system, it's difficult to recommend anything. And even then, there is a lot you can't tell from a picture in terms of the adapters you need.
But hardtube is not that different from tubing.