I'm not an expert here, but I think there could be a bit problem with the rate used to buy back service. To the best of my knowledge, there is no option for a 'holiday', so if you drop out and rejoin the scheme later, you will be doing so at the rate that applies at the time of rejoining, which may well be considerably higher than the rate you are paying now.
Check it out carefully with your HR dept, and consider the possibility that the rules may also have changed by the time you come to rejoin - perhaps the scheme for buying back years could even be closed to new entrants completely.