we have a small playmobile farm, bought years ago.
I find that the kids don't really play with the farm building much, they play with the bits, so over the years our farm has acquired about 4 tractors, loads of extra animals, some extra (not playmobile) fencing to make fields and so on. I would be tempted to buy a smaller farm and then buy lots more animals etc so they have more to play with. Tractor plus trailer for the animals is very popular.
My dd loves the tiny boxes, one girl plus her horse and a bucket, they make great add ons to the farm as it gives them more figures and animals to play with.
As to the person who said 'what do you get if you only spend £20'
well, as I said up thread, we spend about £50. So dd will be getting a scooby doo van that she has been going on about (£20 Asda), plus extra scooby doo figures bought on ebay. Plus other bits and pieces that she wants.
If you have the money to spend, great, but I find it really sad that people go into debt in order to buy a larger item, when it isn't actually necessary. Ds last year got the large lego boat, about £50 new, bought in ebay for about £30. He loved it. They just don't expect to get the large £100 lego sets.
I think it is really important to do Christmas according to your own budget.