Best thing I did was hire one to try it out -then you know both if camping is for you, and what type of camper and what size you need.
Second best thing I did was not have a small VW camper with kids! There is no room to store things, cold up in the roof, no room to dress, no space to cook etc. You have to get out the camper to move things around...You really do need to rent to see if it can work for you, and consider that they are only going to get bigger. I cannot understand how people can do a week in a SWB or LWB VW Transporter with kids and a dog.
Third best thing was buy one ;-) I have a high top LWB Transporter - but it is just me, occasionally a friend. And for that it works.
As others are saying, it is not cheap. Purchase (even with slow depreciation) is large, then £365 road tax, £550 insurance, £80 breakdown, it only does 35mpg on diesel, £250-350 servicing, then you have site fees which vary from £20-60 a night (more for a family I would think). Do look at vans that are not VW, there is a huge 'scene tax' on them and you can get other brands at 20-30% less, and they are better vehicles. Add it all up and value is not a reason to buy one/.
But flexibility is. Mine lives most of the year packed and ready to go - I have all my bedding, some toiletries and dry foods etc in there, I literally fill up the water tank, pop in the fresh food and my clothes, and go...Last year (First year of ownership) I did 58 days in the thing at weekends and main holidays, let alone a huge number of cuppas at the end of hillwalking days or overlooking a beach.
But then I am single, kids old enough to have left home and have most weekends clear, and the ability to go with the weather on holidays. I only cook for one. I would not have had that flexibility when the kids were younger. I also live up north where an off-campsite stay is easy out of season...