I bought my 2L flask from Cream Supplies �ã9.59 (incl VAT) and shipping is another �ã4.19. Don't know how the quality compares to others but I have no complaints.
As Monkey says, it depends on the style but I reckon on about 2-2.5g for an ale and around 3.5g for a lager per 500ml bottle.
Try one of the online calculators like this one https://www.brewersfriend.com/beer-priming-calculator/
The first time I cold crashed 23L I reckon about 0.5L of Starsan and water were sucked back into the FV. Now I take the blowoff tube off and put a loose cover over the blowoff barb on the FV before I start the crash.
That looks very similar to mine and I can see others on ebay at a lower cost than the particular one you linked to. However, the cheaper ones are from China so it depends how quickly you want to get on with the build.
As for other options, I don't know. These are the only things I have...
Previously, for another project, I used a potentiometer with a PC fan but found it had problems: the fan does not turn on until you turn the potentiometer quite far so all of the speed control is limited to maybe the last 20 degrees of the spindle's rotation which makes it very difficult to...