alone or with a turist agency?

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Hi,
my boyfriend and I are planning on doing the Annapurna circuit trek. We are wondering, wheter to book the trip at a turist agency/treking company (here in Slovenia or in Nepal) or if it can be planned and done on our own. We are both in good physical shape but we are not very experienced mountain climbers. I've heard the best time to do it is in the fall (October, November). Can it be done in the spring as well (March, April, May?)
thanks for your help

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