Hello Sappy
I bought a shop with existing IF membership a couple of years ago. All pretty much as treen explained. Although I didn't have to make up any test pieces, I assume because the original staff were staying on and I am qualified as well.
We didn't have a break at all, I took over the shop and IF carried on as usual, they didn't send me on the new members course straight away, it was a couple of months later, by which time I felt it was a bit pointless, as I had shadowed the previous owner for a month before taking over the shop as well.
The membership application form/essay is about 20 pages, if you phone IF and explain you might be taking over a shop they will be able to answer all your questions about fees etc, I can't remember exactly what I paid at the time.